Rework Theme Settings panel

[clembs]

• Proper UI, dropdowns and menus for each setting
• Control avatar size, corner radius & border width/color
• Independant navbar settings
• Border patterns
• Custom cursors
• A11y warnings
• Shadows for widgets & inputs
• Global accent color (maybe, for some specific inputs (or repurpose the primary buttons' color))

Maybe later we could have [i got distracted and now i forgot what i wanted to type]