The first bit of CSS makes the background the blue, and the second bit of code fixes some blue that shows through in the timeline otherwise.
You can simply copy and paste the code into the Stylish Edit page and hit the Save button. That’s all there is to it.
Of course, you can make the background any colour you like by changing the hex code (the number with the # sign) in the first section to another colour code. You can find colour codes with this colour picker. If you know CSS code, you can make other changes to the Twitter background, but I choose to keep it simple for maximum capability.
Once you’ve got Twitter exactly as you want it, you can move on and make changes to other popular sites you use as well. Stylish can use pre-defined styles that you can find saved at userstyles.org.
Whatever colour or styling you choose — have fun!