Fix "GUI Style" page for 3.0

[cspotcode]

https://api.arcade.academy/en/development/programming_guide/gui/style.html

Dupe of #1101? I'm not sure

[cspotcode]

@eruvanos I'm not sure who's best for this, so I assigned to you.

[pushfoo]

The original issue is large and unwieldy. This makes the scope clearer, so I wouldn't call it a dupe.

I'd suggest changing the title and contents to be something like "add basic information on styling the UI."

[eruvanos]

Thanks for the poke, I should be able to work on these issues in around a month. Help always welcome of cause :)

[eruvanos]

First docs are pushed