Thee videos are a good primer - unfortunately they're not anywhere close to answering the question.
I'm looking for the specific property on the DocumentPresenter control that allows me to determine (in the theme) whether or not the document is currently active.
I've got my theme already set up to use the mouseover / mouse click states properly, but I can't seem to find the property that controls whether or not the tab is selected.
Here is my current theme - if you save it to a file and load it up for the DocumentPresenter control you can see it in action - and note how the tab buttons light up when you hover over them, but they don't give an indication which one is actually currently selected.
EDIT: Ok, found the proper trigger: "Telerik.WinControls.UI.RadTabStripElement.IsSelected". Add a condition to check that value for true/false and it shows up. Yay! :)