I did not discover that "Delete" template really means delete in debug mode until it was too late. I also did not know that customizations in debug mode become the default templates in non-debug mode. How can I re-import the default style and start from scratch? I tried importing install/data/templates.xml, but it doesn't work ("The provided file is not valid style XML.").
I realized that I can copy the actual templates out of templates.xml, but this seems to be a very hacky way to do it, and for templates that I deleted whose names I cannot remember, this method will not work.