feat: support rootStyle: false option

[khushthecoder]

Description

This PR fixes an issue where the rootStyle option was being ignored when using codeToHtml with a single theme (e.g. github-light).

Previously, codeToTokens returned rootStyle: undefined for single-theme usage, which unintentionally overwrote the rootStyle passed in options when calling tokensToHast inside codeToHast. This resulted in the default background color (often white for light themes) being forced, even when the user tried to disable it with rootStyle: false or rootStyle: ''.

The fix involves updating codeToHast to only use the rootStyle from codeToTokens if it is defined, otherwise falling back to options.rootStyle.

Linked Issues

Fixes #1175

Additional context

Verified using the reproduction script provided in the issue.

• Before: style="background-color:#fff;color:#24292e" (forced background)
• After: style="" (when rootStyle: '' is passed)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.08%. Comparing base (5ddea95) to head (93d0cbf).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1184   +/-   ##
=======================================
  Coverage   95.08%   95.08%           
=======================================
  Files          91       91           
  Lines        7866     7868    +2     
  Branches     1662     1666    +4     
=======================================
+ Hits         7479     7481    +2     
  Misses        381      381           
  Partials        6        6           

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