fix: use high specificity selector for theme vars

[farnabaz]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Use html:root instead of :root for root selector.

resolves #339

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

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[Tahul]

Excellent, thanks Ahad 👌