Move super sidebar theme mixin to super sidebar styles

What does this MR do and why?

Rather than compile separate stylesheets for each theme, update super sidebar CSS custom properties from .ui-* root level class added for each theme.

Changes

• Remove user_application_theme_css_filename and stylesheet link in head
• Remove theme_helper.scss
• Remove theme_*.scss files
• Add --super-sidebar-active-indicator CSS custom property
• Add --super-sidebar-context-header CSS custom property
• Remove --super-sidebar-primary CSS custom property

Screenshots or screen recordings

There should be no visual changes, screenshots as visual validation

Before	After

How to set up and validate locally

1. Update themes in http://gdk.test:3000/-/profile/preferences
2. Review sidebar styles for each theme

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.