Problem with background in 4K monitor

Schreckgespenst

I have a problem with the background on my homepage oscars.doobacco.de

It don't scale with 4K monitor. So I used this but it doesn't work. Is there a costum.css that I can use. 

html {
background-image:url(http://oscars.doobacco.de/wp-content/uploads/2017/04/backgrounds-2-red-lagoon.png);
background-repeat:repeat-y;
background-size:100%;
background-position:center top;
background-attachment:fixed

}

Thanks for help

Bryan

Use

background-size:cover !important;

that shoud help.

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Schreckgespenst

Thank you.