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the EV_STOCK_CLOSE icon is now used in only one place in the code. I'll provide a MR to remove it entirely.
make the close button round like in all application windows
darken the background of the button on press (like every button do) and lighten on hover
don't use the annotation's color for the window close icon (cross) but use the most appropriate one between black #000000 or white #FFFFFF
(reminder for myself: for an other MR, remove the custom made resize buttons and just use GTK window resize feature)