Description

Allow hosts to specify color scheme preference (light/dark/auto) when loading embedded checkout, enabling visual consistency with host apps.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)
• Documentation update

Is this a Breaking Change or Removal?

If you checked "Breaking change" above, or if you are removing any schema
files or fields:

• I have added ! to my PR title (e.g., feat!: remove field).
• I have added justification below.

## Breaking Changes / Removal Justification

(Please provide a detailed technical and strategic rationale here for why this
breaking change or removal is necessary.)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules