where can i go to start changing background colors and such

my numa numa video is very poggers but i need more customization options

You can go here to put custom css code into your profile. You can also reach this page by navigating to the Personal tab > Dashboard > Profile

You'll need the profile section actually on your profile for customization to work btw. ^^

You can add it like any other section and edit it too directly on edit profile page, tho it's definitely easier to see on the page linked above.