help with how to change the layout of my den

i could easily make the banner for the background myself (I have a banner making app on my phone i can use), but i don't know how to work with css stuff. i'm not even sure what css is.

What are you referring to here? like the background of your page as a whole, or the inner boxes? or did you mean just like an image to go onto your page, if it's not for a div box or anything embedded into the site itself then you won't need CSS, you'll just need the HTML codes, which are already provided by the site

the background as a whole.