feat: accept other theme variables

[hyochan]

Description

feat: accept other theme variables in a custom theme.
test: import theme package locally in the storybook for live debugging.

Test Plan

Change logic in the theme package and test in the storybook and see if it applies dynamically.
Try to add a custom theme and check theme variables are merged correctly.

Related Issues

N/A

Tests

Ensure not failing.

Checklist

Before you create this PR confirms that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• Run yarn test:all and make sure nothing fails. You can run yarn test -u to update snapshots if needed.
• I am willing to follow-up on review comments in a timely manner.

[codecov]

Codecov Report

Merging #214 (cf50777) into main (7c0c829) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
+ Coverage   86.28%   86.54%   +0.26%     
==========================================
  Files          27       27              
  Lines         707      721      +14     
  Branches      292      304      +12     
==========================================
+ Hits          610      624      +14     
  Misses         97       97