Fix dark mode scrollbar

[bcotrim]

Related issues

• Fixes STU-1426

How AI was used in this PR

AI identified the root cause and wrote the CSS fix. Changes were visually verified by the developer before/after.

Proposed Changes

Note

I wasn't able to replicate the issue mentioned in Linear, but I still improved the layout of the scrollbar.

• Add color-scheme: dark to the dark mode media query so the browser renders native UI elements (scrollbars, form controls) with dark styling
• Replace hardcoded color values in .sites-scrollbar with existing CSS custom properties (--color-frame-scrollbar-thumb, --color-frame-scrollbar-border, --color-frame-scrollbar-thumb-hover) for consistency with other scrollbar styles

trunk	now

Testing Instructions

• Switch Studio to dark mode (Settings > Appearance > Dark, or set macOS to dark mode with Studio on "System")
• Navigate to the Settings tab for any site
• Resize the window to trigger scrollbars
• Verify scrollbars match the dark theme (no white background)
• Also check the sidebar site list scrollbar and any modals with scrollable content

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing e581556 vs trunk

app-size

Metric	trunk	e581556	Diff	Change
App Size (Mac)	1249.54 MB	1249.54 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	e581556	Diff	Change
load	1931 ms	1942 ms	+11 ms	⚪ 0.0%

site-startup

Metric	trunk	e581556	Diff	Change
siteCreation	8188 ms	8186 ms	2 ms	⚪ 0.0%
siteStartup	4833 ms	4843 ms	+10 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[epeicher]

Thanks @bcotrim for improving this! I have tested it, and I can confirm that the scrollbars colors are better now. LGTM!

Screen	Before	After
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