I don't recommend add-ons that invert colors as these don't work well on some
websites (and might become unusable in future releases as well if they aren't
maintained).
My advise is to find a dark (but not too dark as black background is not
fully compatible with some websites) system GTK theme (I'm using Arc-Dark)
and then to select in your browser "use system colors". You can customize
some additional settings such as the color of text/visited/unvisited links.
I'm using a dark grey background so that black text (if presented by the
website such as this one for icons such as "Attach") is also visible. Of
course this means that all programmes will now use this theme so if you only
want dark text in the browser then this is not ideal and the add-on method or
dark theme setting in the browser (if available) is preferred.
GTK themes can be used in Qt applications as well (after editing some text
files), so Qt applications without a dark mode can be made to look almost the
same as GTK ones.
