I would like to target the whole of the main content pane in Firefox in order to overlay a visual filter on top of it. Although websites can be altered with scripts and Firefox offers colour options in its preferences, these are imperfect workarounds that only target specific elements of the websites. Instead, I would like to overlay a translucent filter over the main content pane itself that would allow me to control the appearance (dimness, saturation, contrast) of the entire main content pane rather than individual elements of a website. How would I do that? Which files, which code snippets?