Nowadays android devices have a night mode which autotriggers dark mode in supporting apps. Riot should support that. Given it has light and dark themes, it should just autoswitch it depending on the OS setting.
this would then only leave the choice of which light / dark theme one wants, and i think Tachiyomi does this pretty well:

@bmarty can this be set on a project or milestone?
We have other priorities, but any contribution will be appreciated :)
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We have other priorities, but any contribution will be appreciated :)