For experimental purposes I have Thunderbird v.52 x86 (pre-quantum and with legacy addons) installed on one of my computers (Windows 7 here). Now I would like some GUI elements to have a transparent background color – this is possible with CSS, whether userChrome.css or addon does not matter.
But there is an unexpected negative side effect. If I specify transparency with RGBA scheme or procentual opacity, then title bars of child windows get lost (no joke). The same goes for the about:config setting ui.use_native_colors if it is set to false: