Every save is a change in the theme's history: who made it, when, and which files it changed. Uploads, Theme Store updates and their undo are in the history too.
Compare and restore a file
Open a file's history to see each version and what changed in it. Restore any version: it is saved like an edit, with the same checks, so you can undo it too.
Undo a change
Undo a change to put every file it changed back the way it was. If one of those files changed again afterwards, undo the later change first, or restore the version you want from the file's history.
Creating, deleting or renaming a page template is one change too. Undoing a change that created a page template, an upload included, deletes the template, with the same rules as deleting it: it's refused for a page type's default template, a template one of the theme's styles ships, or a template items are assigned to. Undoing a delete brings the template back, and undoing a rename renames it back, with the same rules as renaming it.
Back to the Theme Store version
For a Theme Store theme, compare the theme with the version it's installed from to see which files you changed, added or removed. Then put chosen files, or the whole theme, back to that version.
The history keeps the latest 200 versions of each file and belongs to the theme: a duplicate keeps it, and deleting the theme deletes it.