The customise options are resonably straight forward. You can play around with them to see what they do. It's only the background that I wish to make a note about.
The two panels are 400px by 500px. Your images will automatically resize if they are the wrong dimensions. I could have made the background an image-option too, but tumblr tends to resize large images. I recommend you use an image-host like imageshack, which won't resize your background image unless you tell it to. Just paste the direct link in by the 'Background Image URL' option. The background-image will automatically center around the box.
Here is a guide for if you wish to create a seamless image:
I also recommend using the jpg file type for your background image. I know it's tempting to use a better-quality type of compression, but it keeps the file size low and therefore takes less time to load the first time someone visits your page. Also if you background is not tiled, I recommend fading off at the edges because people with larger monitors will see the edges of your jpg.
