After installing some packages (unfortunately I can't remember which ones) and installing some GTK3 themes (Faience / Faenza), my terminal's menubar and the right-click menu (in some contexts) started to look very ugly !
1: Right-click menu
2: Terminal menubar, notice the white rounded corner rectangles, it looks WAY uglier on other themes
Note that the menubar isn't "ugly" in most contexts, but it still is on the desktops and some applications. (For the menubar, I think the terminal is the only one concerned by this)
I tried going back to my original theme, it didn't fix the problem.