Just about all the stuff you see on your UI are under "/usr/share/", it goes for both applications you install and the SFOS components that make up all the niceties of the device.
Just doing a "ls -l" there shows you pretty much... and find/grep gives you a lot of nice places to start hacking

QML is so tweak-friendly, you can immediately see the results of modifications. And if you botch something, it is easy to restore, just remember to have ssh available all the time you tweak something

UI tweaks, for example require you to restart lipstick after modifying the QML's, just "killall lipstick" is enough to activate your new file.

Looking at that, you will appreciate how hacker friendly the device really is, you can accomplish so much just looking at the QML's and making a small change here and there