Having a custom stylesheet for the rendering of the wysiwyg editor would be awesome. I love how the feature works for the rendered markdown and Jopin-wide app.
As a side effect, this feature may also enable another capability I've longed for -- the ability to increase the default font size in the wysiwyg editor. Right now I'm zooming-in the entire app, which works, but needlessly takes up screen space with larger menus when all I really need is some bigger text for my aging eyes.