Allow userChrome.css to read variables set by WebExtension add-ons

[Matiasf]

setCSSVariable is a javascript function that sets CSS variables so that style sheets can respond to dynamic values.

I request that the variables that are set by this function can also be read by the userChrome.css file.

While my use case is very particular, I can see how this would enable many other extensions to customise the interface for their users, thus making them more powerful and easier to use.

[Jon]

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