I have over 100 themes installed. It's maybe a bit of an obsession, I admit, but it gives my users choice.
The problem is that it's a royal pain to scroll through them. I've thought about trying to edit themes.php to add a simple link at top, with each letter of the alphabet, and using anchors within the page (e.g., the "A" link would just 'link' to #A, scrolling down appropriately).
The problem is that the current code doesn't seem well-suited to this. I wondered if anyone has ever done this before, before I go mucking with the source. :)