STEP 1: Go to your blog and click the customize button or click here.
STEP 2: Click EDIT HTML. Here you see a bunch of crazy code which you need to delete, this is the code of you current theme. Select them all and delete it. You can select all text by holding CTRL+A for pc-users and for mac-users you hold CMD+A. Make sure the box is completely empty!
STEP 3: If you already have the code you can skip this step. If you don’t, you can find the theme code by clicking on the picture of the theme you want above and it will bring you straight to the install code.
STEP 4: Paste the code in the empty ‘html’ box where you deleted the last code. Don’t forget to click on ‘Preview’ first. That way it updates the code and then ‘Save’.
STEP 5: Click on save and reload your page. It’s important that you reload the page, so please do that. Since tumblr build a new themepage you have to reload the webpage before you can see the full theme. The theme is installed and you can now change the colors, add links and upload a background image. I try to add a lot of options so check those out. Enjoy my theme!
For a more detailed explanation with guided images click here!