Hi :-)
First of all, a big thank you to the OpenToonz team for bringing us this great piece of software! Also a big thanks to all of you who spent your weekend working on guides/helping others out and getting OpenToonz running on Linux! :-)
As some of you have probably also stumbled across, OpenToonz uses Qt stylesheets for theming the user interface, and the options in the Preferences -> Interface -> Style combobox correspond to folders in C:\OpenToonz 1.0 stuff\config\qss (I don't know where the Mac version creates this folder).
I figured that while waiting for user guides, I'd take a crack at making the program feel a bit more modern. (the included themes felt a little dated, and the fonts bugged me) I ended up spending the day working on a theme I called Mori. It looks like this:

Note that this is unfinished and only tested on Windows 10. I've covered most views, but decided to call it a day since some UI styles (like panel headers), and certain element sizes are hard coded into the program. Should be complete enough to use in case someone wants to take it for a spin (note that the Xsheet page is the exception, it is only partially styled).
Hope it might be of use to somebody. :-)