[teamleaderleo]

Fixes the base red that #20561 introduced: webviews/test/theme-colors.test.ts "a migrated stylesheet takes its colors from the theme" failed at the feat tip, because composerStates.css:28 mixed the light composer surface toward a literal white.

• New shared token --cmux-tone-light in pages/shared/desktop.css: the light end of the tonal scale. It is the one place that keeps white, marked theme-fixed, matching native ThemeTokens.opaqueOverlayFill, which mixes a light page toward .white.
• --acpmux-composer-surface now mixes toward var(--cmux-tone-light). Light themes paint the same pixels; the dark branch is unchanged.

Testing

• Red: bun test test/theme-colors.test.ts fails on feat tip (coordinator's base-red report at c331694, composerStates.css:28 literal white).
• Green: the same command on this head passes, 7 pass, 0 fail.
• No visual change: the computed color is identical (#fff → #fff through a variable), so no recording.

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes the theme-colors test failure from #20561 by lifting the light composer surface toward a shared theme token instead of a literal white.

• Adds --cmux-tone-light in desktop.css, the single theme-fixed place that keeps white, matching native ThemeTokens.opaqueOverlayFill.
• --acpmux-composer-surface now mixes toward that token; light themes paint identical pixels and the dark branch is unchanged.

^{Written for commit d4f0529. Summary will update on new commits.}

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[teamleaderleo]

Bead cx-edrgf created for this PR by the cmux coordinator.

[blacksmith-sh]

Found 3 test failures on Blacksmith runners:

Failures

Test	View Logs
test/composer-capture.test.ts/composer-capture.test	View Logs
test/composer-search-files-key.test.ts/composer-search-files-key.test	View Logs
test/new-tab-add-project-layer.test.ts/new-tab-add-project-layer.test	View Logs

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[github-actions]

all web/size checks green

[github-actions]

Passes: CI passes on d4f0529f06.

CI passes on d4f0529f06 (run 38602744855 attempt 2).

Written by scripts/ci/classify_failures.py (ci-failure-attribution.yml); signatures are its SIGNATURES table. A machine verdict is the runner's fault, not this PR's; yours means the failing file is one this PR changes, also red on main that main's latest full suite fails the same way, seen on other PRs that it failed on another pull request's run lately.

[github-actions]

CI fast guards passes on d4f0529f06 (https://github.com/manaflow-ai/cmux/actions/runs/38602744855). This covers only the fast guards, not CI: CI's result is the ci-status check, and the CI failure attribution comment names any failing test.