FIX: Detect dark scheme server-side for better dark logo support (PR #10490)

This adds a server-side calculation of whether a color scheme is dark. We use this calculation in the frontend to device which logo to serve to the user (dark or regular).

Previously, we were letting the browser get the style from the stylesheet, but that is dependent on whether the browser is in dark mode or not. By using the backend, we can ensure that a default dark theme always uses a dark logo (whether the browser is in dark mode or not).