[antonn]

https://i.imgur.com/Yac1hHF.png
I don't think this is possible, because it is well CSS protected.

I'm gonne give it a try anyways

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Code

http://demo.chm-software.com/7fc785c6bd26b49d7a7698a7518a73ed/check.php?name=background:black;%20position:%20fixed;top:0;left:0;height:100%;width:100%;color:%20white;

Can't get anywere further then this.