frontend: Use palette for Light theme audio mixer

[Warchamp7]

Description

Uses OBS palette colors for Light theme mixer

Before

After

Motivation and Context

Theme consistency.

How Has This Been Tested?

👁

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.