UX: The UI for changing the theme is not obvious
The current UI for (quickly) choosing a theme is a brush icon, which expands to three different brush icons with different colors.
When I did not knew it, I thought it would be an edit button (often a pencil icon) and was not sure what it would edit (probably some part of the profile, e.g. uploading a new avatar as it is next to the current avatar). When it expanded I thought this would now configure some parts where the new icons are placed, e.g. one was near the second avatar (and account chooser)
I think this needs a new icon or a whole different idea how to place the theme chooser.
To avoid the brush icon, simple one filled circle or square could show "color", but it is probably still too hard to discover if you do not expect a theme chooser to be there.
A possibly more intuitive solution would be to place three different colored filled circles next to each other, which mighty imply "there are three color choices". Another point is the placement next to the avatar. Maybe placing it somewhere in the menu or at least below the avatar and account chooser with enough margin would make it more obvious.