Site Redesign / Theme Style Selector

Allow users to choose from multiple UI themes beyond the current dark/light/soft options. Add a full style selector with different colour palettes (e.g. midnight blue, forest green, warm sand, high contrast) and layout density options (compact vs comfortable). Consider a broader visual refresh of the platform — updated card styles, refined typography hierarchy, smoother transitions, and more polished empty states. Could also include per-company custom theming for enterprise customers.