A few helpful Tumblr theme tutorials...
A couple are actually scripts or widgets but they’re helpful nonetheless.
Tumblr Tag Clouds by Heather Rivers. There are other tag cloud generators for Tumblr but, in my experience at least, this one is the easiest to set up and style. And, it’s super simple to add to custom themes.
Tumblr Infinite Scrolling Instructions by Cody Sherman. Another easy to install script with really detailed instructions. I don’t often use infinite scrolling but when I need it, this works like a charm.
How do I get large images in my Tumblr custom theme? This one is from Stack Overflow. The maximum size for images on Tumblr themes is 500px but the second answer on the page by Fred Stevens-Smith offers a great workaround for displaying high res photos instead.