Provide dark theme

[Miepee]

Currently, the Matrix Public Archive only supports a light theme.

Hydrogen already supports a dark theme. It would be nice if the archive would provide an option with auto, light, dark too.

[MadLittleMods]

I'd be tempted to only support auto light/dark theme to avoid the flash of light theme on every page-load since we would have to read the preference out of local storage. Or maybe there is a better standard way people do these things? (can't use cookies because we want the responses to be cached)