Theme selection has been added to the web app! You can now select your theme via the side panel (top left button, bars icon). The two themes are default (the old theme) and flat (the new default). I know someone will say, why not call the new default, default. Well, I am referring to jQuery Mobile default and I also don’t have a good short name for it.
Update: I am noticing the quick flicker between themes if your selected theme is default, on app startup. I am working on a fix now.