Default Windows 10 Themes contains the three default themes that come with Windows 10.

Having these can be handy if yours are missing or if you're having issues getting to Theme settings.

This ZIP file contains aero.theme (default), theme1.theme, and theme2.theme.

These themes should be located in C:\Windows\Resources\Themes and aero.theme should also found in C:\Users\YOUR USERNAME\AppData\Local\Microsoft\Windows\Themes. Note that AppData is a hidden folder by default.

You can extract these to those folders, or double-click them from anywhere to open Themes.