Several months ago we transitioned the visual design of the app (in effect) from dark to light. The reason for this was mostly consistency across the app — we also had a logic that elements that move horizontally, the pulse graph for example, should be dark elements; ones that scroll vertically, such as your home feed, should be light. We weren’t really following this in the previous design and now it’s something that is consistent across the app.
In regard to themes, we don’t have any plans to introduce them to Monzo yet. As Naji says, our primary focus now with the visual design of Monzo’s apps is that understanding your money at a glance is as simple as possible. Maybe it’s something we’ll consider, but realistically I doubt it’ll be sometime soon.