You can't modify its UI CSS, because it's an <embed> frame.

There's a way, but it requires more tweaking.

That's one area I had problems with myself. I could change the chat just fine myself.

It's embed in the html file and has another css file itself so there's technically two css files for one page. I'm not sure myself, but I think this is what it is then.

https://www.w3schools.com/tags/tag_iframe.asp

I never use iframes myself. Not sure how to do this myself even. iframes usually aren't preferred for many / most people though. Maybe the link above will help if there is some way to do it.

edit

It would have to be linked to this page somehow to be able to edit it. It's one page, but two CSS files - one for a sub then one for pic8.co.

It would have to stand on it's own at that point to as well or else everyone would be editing the same page at that point. Be a cruel joke to have 100s of people with the ability to edit the same page and it's always changing. I should start a website for this one thing alone.

https://pic8.co/

Just paint your screen the desired color!