Defining background color
Twitter blue is nice, but since my twitter account is related to my blog Cats Who Code, I thought that it should be a lot better if I use the same color scheme as CWC uses.
To do so, get the hexadecimal code for your site background color, and then go to twitter settings.
Click on the Design tab and fill the form with the colors you want.
Photoshop design
It's time to be creative and design a sidebar image in Photoshop (or The Gimp). I started by taking a screenshot of my current Twitter homepage, so I can use it as a guide in Photoshop and know exactly where the blank parts are.
Once you have done your design, simply save the image.
Import the image in Twitter
Twitter allows you to use your own background image. You just have to go to twitter settings and click on the Design tab. Choose the image, and you're done.