Even though I am relatively new to Blender I am recording a tutorial for a Blender exporter. I have been advised that I should use the Default colour scheme (for the UI). I have no idea which one that is. Can anyone tell me?
You can also save a preset called "Default", so that You can switch back to it from the Theme Presets Menu. Since you lost the Default theme, You can follow the answer as posted by @jasperge and then the steps below
Add a new preset based on factory default theme
Name the new preset as Default
Later, if you changed your theme to some other theme, You can switch back to Default very easily
Arguably just as important (more important?) as the default color scheme is the default layout (keybindings, screen layout, etc.). You might be better off resetting the whole thing to defaults by going to File > Load Factory Defaults.
Alternatively, include a start-up .blend file (and theme) as a downloadable addition to your tutorial. At the very least, this gives your audience the chance to have their whole set-up match yours right from the start.