Ripping Themes

To rip themes, right click anywhere on the myBB forum and copy the .css themes and copy them to your stylesheet.

Make sure your version of style is similar to the ripped theme so you don't get random codeposts popping up everywhere.

Global.css is usually the one that determines color and index page templates, so enjoy. Just replace your .css with the ripped one and you should be good to go.

This could be useful if I ever decide to start a forum and steal a theme

It's not that simple to rip a complex theme. You need to read some of the source as well to get the header links etc.
You would also need to rip every single image.