feat: add CSS variables

[MartinCupela]

🎯 Goal

Add base variables for the SDK styling.

[github-actions]

Size Change: +5.8 kB (+2.04%)

Total Size: 290 kB

Filename	Size	Change
./dist/css/index.css	5.9 kB	+5.8 kB (+5573.08%)	🆘

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/Channel-pqo4jV1s.js	24.4 kB
./dist/ChannelPreview-BNyNO58P.js	17.8 kB
./dist/cjs/emojis.js	3.29 kB
./dist/cjs/experimental.js	5.8 kB
./dist/cjs/index.js	165 kB
./dist/cjs/mp3-encoder.js	1.27 kB
./dist/css/v2/emoji-mart.css	1.84 kB
./dist/css/v2/emoji-replacement.css	300 B
./dist/css/v2/index.css	39.4 kB
./dist/css/v2/index.layout.css	22.8 kB
./dist/MessageInputContext-Buw2XInb.js	314 B

_{compressed-size-action}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.11%. Comparing base (c249ef7) to head (a9595a0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2913   +/-   ##
=======================================
  Coverage   80.11%   80.11%           
=======================================
  Files         506      506           
  Lines       10777    10777           
  Branches     2709     2709           
=======================================
  Hits         8634     8634           
  Misses       1890     1890           
  Partials      253      253           

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