feature: チャンネル投稿はフォームに色がつくとうれしい

[fruitriin]

What

投稿先のチャンネルに応じてPostFormに色が付きます
https://github.com/misskey-dev/misskey/assets/18308639/f20abc02-708b-4f4c-a70b-b82ea885abd7

Why

投稿先の誤爆を減らすのに役立ちます

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (272dc20) 79.97% compared to head (a934881) 99.37%.
Report is 14 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #12686       +/-   ##
============================================
+ Coverage    79.97%   99.37%   +19.39%     
============================================
  Files          779        4      -775     
  Lines        84662     4476    -80186     
  Branches      7901       58     -7843     
============================================
- Hits         67710     4448    -63262     
+ Misses       16952       28    -16924     

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[syuilo]

👍👍