I saw that you asked for some examples for this, so maybe I'm asking this at the wrong time since it sounds like you are looking into a new design already. 
I wanted to ask how the theme in the app is done? I can't seem to find a CSS file for changing the background colors on the notebooks and the notes, etc. Because I wanted to play around a bit with some design updates. Eg. something like Bear, or such.
I think it is hard to look at other apps in some points, since I believe your design is fairly good. For me personally it is more a bit more whitespace, some tweaking and such to improve it. Because looking at Evernote I think it looks good, and I think Joplin can be in the same area! 
Is there a guide or such how I can play around with changing the full theme? Not only the Markdown renderer.