im not using linux, but i notice with opera every time I choose force dark themes on websites, this white strips appear on youtube videos and other web videos when they go into full screen, all i did was turn this option of and it sorted it for me.

Sorry if its late, but if it helps.

@blitzkywitzky its very easy, go to settings / then just type keyboard in search box / then wenter configure shortcuts /
once there delete shortcuts for cycle through tabs / add new shortcuts for switch tabs / done.