As the colors of QToolTip depend on the desktop environment used,
explicitely set background and text color for fixing cases like
white text on bright yellow background.
Closes#3006955.
There were some changes to DefaultStyle (such as styling new
WelcomeScreen or SideBar) which weren't part of CusisStyle yet.
According lines were copied and pasted from DefaultStyle. Probably
colors need some tuning.
The current toolbar backgrounds are just PNGs with a linear gradient
inside. We can achieve the same by using qlineargradient property in
the according style.css.
