Add userChrome.css support

[RaafatTurki]

I find myself hand editing prefs.js after using this project just to add support for custom css into firefox. A good set of rules to enable in that regard are

toolkit.legacyUserProfileCustomizations.stylesheets
layers.acceleration.force-enabled
gfx.webrender.all
svg.context-properties.content.enabled

[allo-]

I see a point for toolkit.legacyUserProfileCustomizations.stylesheets even when the default profile should rather not include every UI setting. I am not sure if the other options are generally useful or something one should either trust Mozilla with or set it as a power user without the help of a tool that is intended to have options that are understandable by a normal user why they should decide for or against that option.