Since it’s just an Electron app, why not allow the full power of CSS instead of (or at least in addition to) trying to recreate a worse version of that in a custom editor?
For example, I’d like to make two simple modifications
- make the avatars smaller and
- change the left margin in a list of mail folders
Now, with a browser embedded picker I can actually select the right element, then google around to find the right margin/padding concoction to do what I need, waste a bit more time remembering the difference between . and #.
In your theme customizer I can’t even select a detailed enough element, only its container. Then I’m presented with a huge unsearchable list of properties which don’t even highlight the UI element they apply to. I can’t type wid and have it autocomplete to width and see the allowed values, I can’t easily google an example from some pro dev on how to style and copy&paste that, I can’t find good docs on the stylig system with examples.
But the worst part is there are no properties for what I need because this is a very reduced styling system.
So you pay the price of an Electron app - slow and resource hungry - but you don’t reap much benefits of it: powerful styling system and scripting