In Firefox we have two options at
Firefox->Preferences->Preferences->Fonts and colors->Colors menu,
Use system colors and
Sites can use other colors.
I would like keep the first one checked (and this is ok) and change the second on a quick way.
A quick way could be pressing a shortcut on keyboard, running a terminal command or changing a content of a config file (because I can do a shell script and use a keyboard command).
My motivation is I would like to always use my system colors but if a webpage has strange visuals, I'd like change it to the original quickly.