feat: add applyTheme helper

[mxschll]

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.59%. Comparing base (1ae7b3f) to head (d28d59d).

Files with missing lines	Patch %	Lines
src/index.ts	92.85%	1 Missing ⚠️

❌ Your project check has failed because the head coverage (79.59%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   78.12%   79.59%   +1.46%     
==========================================
  Files           1        1              
  Lines          32       49      +17     
  Branches       14       18       +4     
==========================================
+ Hits           25       39      +14     
- Misses          4        8       +4     
+ Partials        3        2       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[NathanZlion]

LGTM. I think the codecov thingy is just glaky