A basic custom theme can be developed very easily – just create a graphic 250 pixels wide and 500 to 800 pixels tall. Pick a regular (non custom) Twitter theme that is close to the background color that you are using and then adjust the colors accordingly. Two examples can be found here:
Note that this background is optimized for people who use modern laptops and larger screen monitors, capable of browsing at resolutions better (higher) than 800 x 600. It’s been tested – for example – on a 13″ wide Macbook. It will look good on most Netbooks … since they use a high resolution to fit an entire page on a relatively small screen. Where it won’t look good is on older machines/monitors where 800 x 600 is the default resolution … but fewer and fewer people are using this resolution, IMHO.
For a more elaborate background theme, create a visual 1600 wide x 1200 wide. A background to use as a template exists at this location:
I had nothing to do with this background. It just provides a nice template you can use to get started. In most browsers you can right click to bring up a menu and then save background to save the background image.