I managed to solve the problem by following the steps described by you in this other topic ( No highlight of apps on task manager and application menu when cursor is hovered over ).
I don’t think my tty are working for a reason, but I don’t want to open another embarassing issue.
(F1 is booting screen, F2 is frozen loggin screen if logged out and black frozen screen else, F3 is the “real” GUI… 
Other function buttons doesn’t work. Anyway, it’s not important for the moment).
rm -rf ~/.cache/* worked well anyway, and setting default Plasma theme/colors etc. didn’t change anything.