I learned to code by making a geocities page back in the 90's. I like to imagine that tumblr is the geocities of this generation. Lots of young kids are making cool themes that happen to have dynamic content rather than all static .html files.
This is my throwback to the good ol' days.
Just go to your customize page and paste the theme.html file into the custom HTML box. All of the images here are just for reference, the theme uses versions already uploaded to tumblr.
What's a website without a hit counter? Go to http://www.hitwebcounter.com/ and # for your web counter. It will give you a piece of HTML that has an tag that looks something like this:
http://hitwebcounter.com/counter/counter.php?page=[YOUR-PAGE-NUMBER]&style=0005&nbdigits=5&type=page&initCount=0
Copy the [YOUR PAGE NUMBER] value and when you install your theme paste this into the appropriate appearance option on the sidebar.
For your 21st century hit counter, get your Google Analytics code (something like XX-00000000) and paste that into the appearance option as well.
It looks like this: