Add support for custom properties

[mzyil]

No description provided.

[frvge]

@mzyil , great work! This is a much requested feature.

Could you please have a look at the failing tests? I think with the support for the custom properties, they might be outdated.

Please also try to add a unit test for the detection of the custom properties.

[parak80]

Problem solved, there was a build error when i was using only IE css by writing :ms-fullscreen, :root className{}
It needed - before all of this.
Thank you for you help anyway :)

[mzyil]

@frvge I'm sorry but we moved on from this package, and I don't have much time in my hands to continue working on this PR.