Implement dark mode (#2078)

* User theme settings

* Initial rough pass on colors

* More progress on dark mode

Author: Shpigford

.cursor/rules/ui-ux-design-guidelines.mdc

@@ -15,7 +15,7 @@ The codebase uses TailwindCSS v4.x (the newest version) with a custom design sys
 
 - Always start by referencing [maybe-design-system.css](mdc:app/assets/tailwind/maybe-design-system.css) to see the base primitives, functional tokens, and component tokens we use in the codebase
 - Always prefer using the functional "tokens" defined in @maybe-design-system.css when possible.
-  - Example 1: use `text-primary` rather than `text-gray-900`
+  - Example 1: use `text-primary` rather than `text-primary`
   - Example 2: use `bg-container` rather than `bg-white`
   - Example 3: use `border border-primary` rather than `border border-gray-200` 
 - Never create new styles in [maybe-design-system.css](mdc:app/assets/tailwind/maybe-design-system.css) or [application.css](mdc:app/assets/tailwind/application.css) without explicitly receiving permission to do so