Add documentation for color variables

[overdodactyl]

Add documentation to the wiki for what each color variable alters.

This should make it easier for users to customize Firefox to a greater extent.

[arvenil]

If I would like to make New Tab background completely black which variable I should change?

[overdodactyl]

If you want to change the background of all ShadowFox styled pages (including ones like about:preferences and about:addons), you can adding the following to the colorOverrides.css file:

--in-content-page-background: black !important;

If you want it to apply only on specific ones, you could add something like the following to userContent_customization.css:

@-moz-document url(about:newtab), url(about:home), url("about:blank") {
     :root {
       --in-content-page-background: black !important;
     }
 }

[cinerea0]

Is this issue still being worked on? I think it would be a great addition to shadowfox, especially for people who want more control over the colorscheme of the browser without having to write an entire userchrome from scratch.