Most questions i've read in my ask inbox.
How I can install a theme?
1. First you have to copy the code from the theme that you select
2. Than you have to go back to your tumblr
3. Now click on the costumize button
4. Click on the "HTML Edit" - Button
5. Delete the old Code
6. And instert the new
How I can set a background-image
Sorry, there is now way. At themes that you like we keep an eye on good design and nice themes, so wie don't want that our user use bad pictures as background. But you can set a nice color.
How can I set the Siedbar Image?
Thats easy, go in your theme settings, and set the "Sidebar Image".