stylesheets are css documents. a css document controls the theme of your site, pretty much! here, i'll explain how to link css documents to your html documents. it's easy i promise!

making your css doc:

starting on a new, blank css document, paste this:
body { }

there's some very basic things we can put here. body-color will change the background color of your page. color will change the color of any text in a "p" tag. font-family will change the default font. background-image will change the background image.
background-repeat: repeat; background-repeat: repeat-x; background-repeat: repeat-y; background-repeat: no-repeat;

to change the size of your background, try out this code and play around with the percentage! (under the "body" bracket):
background-size: 100%;