Ok so there are options. If you want something to apply to all the pages, put it in your style.css thing (to go there, go to the main neocities page, hit edit site button and it will show you all of your pages; click style.css) (also is a comment).
Single color:
body { background-color: blue; }
Gradient:
body { background-image: linear-gradient(blue, yellow, green); }
Picture: (in order to get this to work, you have to drag the image into the thing where it shows you all your files, in the edit section (where I said before))
body { background-image: url("you-baddie.jpg"); }
you might have to change some parameters and things
also this is how you make a link:
tasting tastingtasting