refactor(frontend): prefix css variables

[syuilo]

What

Resolve #14620

Why

• 外部ライブラリの変数名と衝突する
• ローカル変数名と衝突する
• グローバルに定義されているテーマ関係の変数であるということが視覚的に分かりやすくなる

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

[github-actions[bot]]

このPRによるapi.jsonの差分 差分はありません。 Get diff files from Workflow Page

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 26.93157% with 662 lines in your changes missing coverage. Please review.

Project coverage is 39.66%. Comparing base (c14eba3) to head (0d29f11). Report is 13 commits behind head on develop.

Files with missing lines	Patch %	Lines
packages/frontend/src/ui/_common_/navbar.vue	0.00%	27 Missing :warning:
...es/frontend/src/components/MkSignupDialog.form.vue	0.00%	22 Missing :warning:
packages/frontend/src/ui/universal.vue	0.00%	18 Missing :warning:
...ackages/frontend/src/components/MkNoteDetailed.vue	0.00%	17 Missing :warning:
packages/frontend/src/ui/deck/column.vue	0.00%	16 Missing :warning:
packages/frontend/src/ui/deck.vue	0.00%	13 Missing :warning:
packages/frontend/src/components/MkFolder.vue	0.00%	12 Missing :warning:
packages/frontend/src/components/MkRadio.vue	0.00%	10 Missing :warning:
packages/frontend/src/pages/admin-user.vue	0.00%	10 Missing :warning:
...ges/frontend/src/ui/_common_/navbar-for-mobile.vue	0.00%	10 Missing :warning:
... and 182 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14725 +/- ## =========================================== + Coverage 39.63% 39.66% +0.03% =========================================== Files 1553 1553 Lines 195043 195037 -6 Branches 3607 2574 -1033 =========================================== + Hits 77297 77368 +71 + Misses 117144 117101 -43 + Partials 602 568 -34 ```

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[syuilo]

レビュワーを募集しております

[kakkokari-gtyih]

--radiusとか--marginとかにはprefixつけない？

[syuilo]

テーマと関係ないものは付けてない

[syuilo]

別のprefixはつけて良さそう

[syuilo]

別のPRにする

[syuilo]

マージするぞ

[syuilo]

approveを募集しております

[syuilo]

🙏🏿