Which GUI toolkit should I learn so that I can "Hack the UI"

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**Opinions invited**: I want to work on the UI, beyond small issues, 
like creating custom widgets, notebookbar, tabbed UI etc. I feel that I 
need to know a GUI toolkit well, like gtk or qt for that. So some days 
ago, I started playing around with GTK. My question is that which 
toolkit will be more useful? I picked GTK because we use .ui files for 
the UI, and being good at GTK will somehow help when working on LO UI, 
but again I don't know, that's just an idea.