Context: https://meta.discourse.org/t/introducing-discourse-splash-a-visual-preloader-displayed-while-site-assets-load/232003/17
We currently set the theme secondary color as the background for the splash, and this works and respects light/dark modes.
The issue is that we set it on the #d-splash div. That div doesn't have a specified height and only gets its height when the splash image loads.
This can cause a flicker effect where the <HTML> background shows for a fraction of a second while the splash image loads.
This PR sets the theme color on the <HTML> tag to alleviate this. This allows us to set the theme color a little bit sooner and should hopefully prevent the flicker effect from happening.
This PR also adds the theme-color <meta> tag for dark mode. Browsers that don't support multiple theme-color tags will ignore the second tag and fall back to the first one.
3d9464b7da