Reset Defaults in New Customize Page 

I’ve had like 30 messages all about Reset Default not working in the new customize page and there’s nothing I can do about it as it’s a tumblr problem. If it’s annoying you as much as it is me, try contacting tumblr and let them know we need either the classic one back or at least for the new page to work correctly (I’ve pretty much spammed their inbox every time I come across something new that doesn’t work).

For the time being, I have sort-of found a temporary method that can work and it should basically help reset everything. It requires you to edit the HTML slightly, so if you’re not familiar with HTML/CSS, I would suggest you try other methods first.