Using your left mouse button click in the box below the theme you want to use to copy
Click here or
click Customize appearence
on your Tumblr Dashboard/Tumblelog
Click the "Edit HTML" button in the top left corner of the customize page.
If you have a premade theme installed click "Use custom HTML"
Delete any HTML already there (Right click, select all then Delete) and paste in the code you copied in
Click "Update Preview" then click "Save" then "Close" in the top left corner and your theme is installed!