Fix stylelint errors in theme SCSS

[muskgupta]

Resolves string-quotes, at-import-no-partial-leading-underscore, max-empty-lines, rule-empty-line-before, and missing-end-of-source-newline violations flagged by the RTL Compatibility Check workflow.

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• 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 change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.