frontend: Fix nested menu styling

[Warchamp7]

Description

Fixes a regression in the audio mixer update that broke the styling of nested QMenus

Before	After

Motivation and Context

Want consistent appearance of menus.

How Has This Been Tested?

👁

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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