Great, I will be very interested to see how you go using GTK3. You were, almost certainly using GTK2 originally, its the Lazarus default on Linux.
If you revert back to qt5, there is a utility, qt5ct, that you can install using your distros package manager that will let you alter a number of display parameters for qt5 apps. Things like colors, fonts, sizes, styles etc. Pretty useful if you like fiddling to get exactly what you want.
Davo