I accidentally marked all forums read. How do I undo this? Not trying to post a useless topic or anything.

You can't.

You don't, you've just got to avoid the button in the future. It's not the end of the world if you don't read every post either, open the areas you find interesting and open the topics that interest you. The posts are also sorted by last post date so if you're really that crazy about it just check the time of the latest post (it's on the right) and compare it to the time you clicked the button.