I switch between userContent.css files that force the fonts and colors to be the same on every page.
There are names like
userContent Normal Color.css
userContent Large Color.css
In the Color files, the font colors aren't forced, that's the only difference.
The chosen file would be copied overwriting userContent.css and 'in use' would be appended to its file name.
When switching to and from the Color files, it should also send keys to switch the option Tools->Options->Colors->Allow pages to choose their own colors.
Then it restarts Firefox (there's a menu entry for that available as an add-on).
Additionally I'd like it to run DimScreen when colors are switched on and close it when switching back to no colors.
A hotkey, e.g. Win+Ctrl+1, 2, 3, 4, and shortcuts (.lnk) to switch to any file in the 'chrome' folder, but it'd be ok with just the 4 choices as above.