i no longer know where the original theme for my blog came from, that was like 4 years ago and i remember picking it up in the tumblr theme garden back when it listed user made themes. but the theme i have now only vaguely resembles what it looked like originally and most of it is cobbled together by me.
however, a gradient background in css is p easy. the one i use is
background: linear-gradient(#ffe1ff, #ffffff);
with the hex codes being the colors you want in your gradient