Once you find a skin you like, click the download button.You will be given the option to “Open With” or “Save File.” We recommend that you save the file somewhere on your hard drive where you can easily find it.
Once it is done downloading, locate the file and unzip it.

Sign in to Twitter, and select the “Setting” tab at the top-right side of the page.

When the settings page opens, select the “Design” tab.

Select the “Change background image »” button.

Click the “Browse” button and locate the file saved on your computer.
If the image needs to be repeated, select “tile background”.
Make sure to click “save changes.”

Once the background has changed, click  “Change design colors »”.

Enter the hexadecimal color codes provided.
You can customize your skin further by using the color selection tool.