How to install a theme
Go in your dashboard, and click on settings icon.
You can find your blog name on the left of the page, click on it.
Then search for customize button in the page.
Edit your html code.
Get your html code (example from here: rickerlrthemes.tumblr.com/ribbon), copy the whole code.
Return in your customize page and delete the whole code you see, then paste the new one you copied from my website for example. After then click on Update preview.
Wait till tumblr updates your preview and click on Save.
Refresh your page (F5) and… Congrat! you installed your theme!