Added Themes Tab and Custom Client Themes

[lsheasel]

Summary

Adds a modern theming system (import/customize), auto-applies the last used theme on startup, enforces a single active theme, and polishes the UI (icons, search spacing, profile grids). Includes a demo theme.

Highlights

• Themes tab: import/delete themes
• Theme customizer for your own CSS
• Auto-apply last theme on startup
• Single active theme (prevents conflicts)

Why?

• To customize the client to your own preference
• Not many clients have this

How to use

• Import a theme in Settings → Themes → Import (demo down below)
• Toggle to activate (only one theme at a time)
• Add custom CSS in Settings → Customize (auto-injected on startup)

Notice

• The demo theme is far from perfect. I'm not a UI/UX designer, but I tried my best.
• It only works in dev tests for now. I'm trying to figure out how to make it work in a production build.

Images

Bevor

After

Themes and Customize Tab

Demo CSS

demo-theme.css

[copyandexecute]

thanks for PR, looks interesting