We need rewrite CSS with new palette dense-dashboard flat-solid. Need keep selectors, layout properties unchanged. Can change colors, shadows (to none anyway), radius small 0-6px, transitions, custom props. Must ensure original hardcoded box-shadow header becomes maybe none? flat-solid says shadow none everywhere. But original header has box-shadow. We can change to none (allowed). Transform? Layout? transition allowed. transform translateY? That's visual but not in allowed? It says only colors, shadows, decorative properties may change. transform is not listed as forbidden? But "layout structure" maybe transform hover could be considered visual. Original had transform translateY(-4px) on card hover. We can change or keep? Since shadow none, maybe keep slight transform? But flat-solid minimal, maybe remove translateY? But transform property is not in forbidden list? Actually forbidden list includes display, position, grid, flex, width, height, padding, margin, gap, justify etc. transform not mentioned. Could change to none or keep. To be safe perhaps keep transform but adjust? But target flat-solid: no shadows, flat. Hover could be border color change only. I'll set transform: none? But that changes behavior. It's allowed? "only colors, shadows, and decorative properties may change" transform is decorative, likely allowed. We can set transform: none or translateY(-2px). Since previous attempt didn't change enough, we need distinct palette: slate background #f1f5f9, cards white, accent blue #3b82f6, text #0f172a, borders slate. Use flat solid with no gradients. Radius 0-6px. Need keep custom properties names. We'll change values. Keep variable names. Need adjust all hex codes. Also border-left? Change to border-left: 3px solid var(--color-primary) maybe primary is blue. Could use border-left: 4px solid var(--color-primary)? Border styles/width? Border-width is forbidden to change? It says border-color and border-style NOT border-width. So we must keep border-width same: original card border-left: 3px solid var(--color-primary). Keep 3px. But color variable changes. Good.

Need maintain padding/margin within ±20%. Custom properties values affect layout? --space-section is padding. It is allowed to change? "padding, margin (keep similar values, ±20% maximum)" and custom properties values can change but if they are used for spacing? It says CSS custom properties (--color-*, --shadow-*) values, but --space-section is not color/shadow. To be safe, keep spacing variables unchanged or within 20%. Original --space-section:4rem, --space-card:1.5rem, --space-gap:1.25rem. Could keep same to avoid layout changes. Or slightly adjust within 20%: e.g. 4rem exact, 1.4rem? But better keep exact to comply. The target says compact padding 8-16px but original comfortable. Since layout must remain, don't alter spacing. Only color vars and transitions, radius.

Need ensure all original selectors preserved exactly. Note indentation weird, but selectors same. Need output CSS with same selectors and declarations. We must not remove any selectors or add new. We can remove/adjust properties? We should keep all declarations but change allowed values. Should not remove declarations. For example box-shadow none all. Header box-shadow originally 0 2px 8px rgba(0,0,0,.06); change to none. Good. Card border-left variable color changes. Transition timing can change.

Let's craft new CSS:

:root {
    --color-primary: #2563eb; // blue-600
    --color-primary-dark: #1d4ed8; // blue-700
    --color-accent: #0ea5e9; // sky-500
    --color-surface: #f8fafc; // slate-50
    --color-text: #0f172a; // slate-900
    --rgb-primary: 37,99,235;
    --rgb-accent: 14,165,233;
    --radius-sm: 2px;
    --radius-md: 4px;
    --radius-lg: 6px;
    --radius-xl: 8px;
    --shadow-sm: none;
    --shadow-md: none;
    --shadow-lg: none;
    --space-section: 4rem; // keep
    --space-card: 1.5rem;
    --space-gap: 1.25rem;
    --transition: 0.2s ease-out; // change
    --heading-weight: 800;
    --body-line-height: 1.65;
}

Body maybe background? Original didn't set body background. But palette could add? This changes visual and allowed. Should we add body { background-color: var(--color-bg)? } Need no new variable? We can set background-color directly. But original body selector only color and line-height. Adding background-color would be new property but not new selector. Allowed? The instructions say change all color values; adding background-color is okay? It might be considered "decorative". But if we add background to body, then cards need white background. Original .card had no background-color. We can add background-color: #ffffff to .card. Since original .card lacked, adding allowed? It is a color value. Good. But be careful: They said "Only colors, shadows, and decorative properties may change" and "adding" likely okay. But to preserve exact selectors, properties can be added? Usually acceptable. But to be safe, we can set body background via existing body selector, and card background via .card selector. That's adding properties but not selectors. Should be fine.

Need ensure no gradients. flat-solid means no gradients.

Need update all colors:
- body color text #0f172a
- a color primary blue
- a hover accent sky
- card border-left primary blue
- header box-shadow none
- maybe card background white, border? Original no border except left. Could add border: 1px solid #e2e8f0? But border-style/color allowed, border-width? We can add border-width 1px? Since original didn't have, adding is not "changing" maybe okay. But border-width is forbidden to change, but adding new border-width might be considered changing? To avoid, don't add border. Instead use background-color contrast and left border. But flat-solid defined by color contrast and borders. We can add border if not changing existing? Yet safer: only replace values in existing declarations. We can add background-color to .card selector without altering layout? It doesn't affect layout. Good.

Need maintain .card:hover transform translateY(-4px)?? With no shadow, translateY might look odd but okay. Could change transform: translateY(-2px) or none. I'll set transform: translateY(-2px); box-shadow: none; but box-shadow already none. Maybe transform none? But original had translateY(-4px). Changing transform allowed? It isn't explicit allowed, but it's decorative. Let's set translateY(-2px). New transition .2s ease-out. Good.

Need adjust radius: --radius-md 6px? But target minimal 0-6px. Use sm 0, md 2, lg 4, xl 6? But original uses lg for cards, md for buttons. We'll set sm:0, md:2px, lg:4px, xl:6px. But "minimal radius (0-6px)" okay. Maybe lg 6, xl 8? They said 0-6px. Use 6 max. Card radius lg = 4px? Better card 6px, button 4px. Set sm:0, md:4px, lg:6px, xl:8px? Max 8 violates 0-6. So set xl:6px too. We'll do sm:0, md:4px, lg:6px, xl:6px.

Need update transition: 0.2s ease-out.

Need replace rgba header with none. Since flat solid no shadows. If box-shadow none everywhere, then in header box-shadow none.

Need maybe update media queries: no colors there. Keep same.

Need preserve comments? The original has comments. We can keep or omit? It says output CSS preserving selectors exactly; comments not important. Could keep section comments but maybe no need. But "Only rewrite CSS" likely include same comments? We can keep all comments, but can change? No need. Better output without extra comments to avoid AI markers? Original comments include "CSSVariantEngine..." etc. Could remove? But if removed, not an issue. However selector preservation is key. Comments can be changed/removed. I'd include same comments maybe but with new palette? Simpler output without top comments to be clean. But may not matter. Since output should be only rewritten CSS. We'll include all selectors and commented section labels as they provide structure, but can omit version line. Hmm.

Need ensure @media selectors unchanged. The original in media queries uses !important; keep.

Need make sure `header, .header, .navbar { box-shadow: 0 2px 8px rgba(0,0,0,.06); }` change to `box-shadow: none;`.

Could add `background-color` to body and .card. Also maybe `border-color`? Existing `border-left: 3px solid var(--color-primary)` stays. We can maybe set `border: 0`? No.

Need maybe update `a:not([class])` color and hover.

Need maybe `::selection`? Not present.

Need think if `--rgb-primary` and `--rgb-accent` values are colors; update to rgb values of new palette.

Need no gradients anywhere.

Potential issue: `.card, [class*="card"]` includes padding var(--space-card) which remains same, okay. radius changed.

Need maybe update `--transition: 0.2s ease-out;` from 0.3 cubic-bezier. Change.

Need perhaps set `body` background-color and color. Since original body selector did not set background but we can add. That will improve contrast. Also set `.card, [class*="card"] { background-color: #ffffff; }` but adding property; allowed. Should we also set border? no.

Need maybe set `header, .header, .navbar { background-color: #ffffff; border-bottom: 1px solid #e2e8f0; }`? That changes visual but adds border width. Border-width prohibited? It says border-color and border-style (NOT border-width). Means you can change color/style of existing border but not width. Adding new border width is technically changing border-width from none to 1px. They say border-color and border-style may change, not border-width. So adding border would violate. Better not add border. Just set background-color on header? It's decorative. Could add background-color: #ffffff to header selector. That's okay, not layout. But if header originally transparent, adding background changes layering. Allowed. But we don't need.

Need maybe set `body` background to slate-100 (#f1f5f9) and cards white. Good.

Need maybe keep `section, .section` unchanged.

Need ensure `--space-section` and others stay same. The mobile media query changes --space-section to 2rem etc; must keep because layout. We must not alter. Good.

Need maybe update `font-size` in `.stats-grid .stat-value { font-size: 3rem; }`? Not allowed, keep.

Need maybe update `letter-spacing`? Not present.

Need produce final CSS. Need no markdown fences. Let's craft carefully.

Let's rewrite:

:root {
    --color-primary: #2563eb;
    --color-primary-dark: #1d4ed8;
    --color-accent: #0ea5e9;
    --color-surface: #f1f5f9;
    --color-text: #0f172a;
    --rgb-primary: 37,99,235;
    --rgb-accent: 14,165,233;
    --radius-sm: 0;
    --radius-md: 4px;
    --radius-lg: 6px;
    --radius-xl: 6px;
    --shadow-sm: none;
    --shadow-md: none;
    --shadow-lg: none;
    --space-section: 4rem;
    --space-card: 1.5rem;
    --space-gap: 1.25rem;
    --transition: 0.2s ease-out;
    --heading-weight: 800;
    --body-line-height: 1.65;
}

body { color: var(--color-text); line-height: var(--body-line-height); background-color: var(--color-surface); }
h1, h2, h3, h4 { font-weight: var(--heading-weight); }
section, .section { padding-top: var(--space-section); padding-bottom: var(--space-section); }
.card, [class*="card"] { border-radius: var(--radius-lg); box-shadow: var(--shadow-md); padding: var(--space-card); transition: var(--transition); background-color: #ffffff; }
.btn, button[class*="btn"], a[class*="btn"] { border-radius: var(--radius-md); transition: var(--transition); }
a:not([class]) { color: var(--color-primary); transition: var(--transition); }

/* Section Layout comments same maybe */
.news-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: var(--space-gap); }

.feature-list { display: grid; grid-template-columns: repeat(2, 1fr); gap: calc(var(--space-gap) * 1.5); }

.hero-inner { display: grid; grid-template-columns: 1fr 1fr; gap: 2rem; align-items: center; }

.testimonial-list { column-count: 2; column-gap: var(--space-gap); }
.testimonial-list > * { break-inside: avoid; margin-bottom: var(--space-gap); }

.partner-grid { display: flex; flex-wrap: wrap; justify-content: center; gap: 2rem; }

.faq-list { display: grid; grid-template-columns: 1fr 1fr; gap: var(--space-gap); }

.stats-grid { display: flex; flex-wrap: wrap; justify-content: center; gap: 2rem; }
.stats-grid .stat-value { font-size: 3rem; }

.cta-inner { text-align: center; max-width: 700px; margin: 0 auto; }

.page-main { max-width: 1400px; margin: 0 auto; }

.card:hover, [class*="card"]:hover { transform: translateY(-2px); box-shadow: var(--shadow-lg); }
a:not([class]):hover { color: var(--color-accent); }
.card { border-left: 3px solid var(--color-primary); }
header, .header, .navbar { box-shadow: none; }

@media (max-width: 1024px) {
    .news-grid { grid-template-columns: repeat(2, 1fr) !important; }
    .feature-list { grid-template-columns: repeat(2, 1fr) !important; }
    .partner-grid { grid-template-columns: repeat(3, 1fr) !important; }
    .stats-grid { grid-template-columns: repeat(2, 1fr) !important; }
    .hero-inner { grid-template-columns: 1fr !important; }
}
@media (max-width: 640px) {
    :root { --space-section: 2rem; --space-card: 1rem; --space-gap: 0.75rem; }
    .news-grid { grid-template-columns: 1fr !important; }
    .feature-list { grid-template-columns: 1fr !important; }
    .faq-list { grid-template-columns: 1fr !important; }
    .testimonial-list { column-count: 1 !important; }
    .partner-grid { grid-template-columns: repeat(2, 1fr) !important; }
    .stats-grid { grid-template-columns: repeat(2, 1fr) !important; }
    .cta-inner { grid-template-columns: 1fr !important; }
}

Need ensure all selectors preserved exactly. We added `background-color` property to body and card. That's new property. Fine.

But need consider `--color-surface` originally #fffbeb used maybe nowhere? Actually --color-surface not used in original CSS. We changed. No issue.

Need maybe update `--color-accent` and `--rgb-accent` consistent.

Need maybe update `header, .header, .navbar { box-shadow: none; }` but flat solid = no shadows. Good.

Potential issue: The instruction says "box-shadow: none everywhere" so we ensure all shadow vars none and header none. Good.

Need maybe adjust `--radius-sm` to 0? Original sm 4, now 0. Good.

Need maybe change comment? We'll include minimal comments maybe not. But to preserve all selectors, comments irrelevant. Could include section comments as original? Not necessary. But if we include, no change. We'll include same section comments? Let's keep comments for clarity, but not critical. The final output may start with :root. No top comments. That's fine. The request says "first character = first CSS character" so no leading blank. We'll output exactly.

Need ensure no markdown fences. Good.

Let's think about "Other files already used: editorial-magazine+gradient-rich. This file's specific values (column counts, gaps, hex codes, shadow specs) MUST differ." We are using slate blue colors, different from bronze. Good.

Need maybe "dense-dashboard" target says grid repeat(4,1fr) etc, but original layout grid-3/grid-2. We cannot change grid-template-columns due to fixed HTML. So keep. We can only change colors/shadows. Good.

Need maybe "flat-solid" says solid background colors only, no gradients. We have solid.

Need maybe "minimal radius 0-6px" yes.

Need maybe "Defined by color contrast and borders" we add white cards on slate.

Need maybe "box-shadow: none everywhere" yes.

Need maybe