In the current version [ 1.1.0 ], you can change the the theme for the code-editor.
Go to 'Settings', and under 'Text Editor' use the drop-down menu next to 'Theme' to change it.
As for the general / overall UI of the app itself, unfortunately, they do not support themes as of yet.
However, if you're using Windows, you may want to look at this 'Dark-Themed, Portable Version' of Superpowers by @FagEmul.
Hoped this helped!