A “reset to default” feature (with a strong worded “are you seerious” step) so a theme acts like it was fresh installed, limited to $wpdb->prefix . ‘options’
Themes have a choice of individual options or mods (one option that is an array, all handled for you). The Customizer function parameters indicate which, so it’s not that difficult to use that to affect all of them.
That does mean that only Customizer settings would be affected, and themes in the wild might not use Customizer. Still, if the UI is in the Customizer, it could at least handle what is registered.
Themes in the WP repo follow certain standards. One is they have to use the Customizer. Another is they have to use only one entry in the option table, so most use theme_mods. Therefore, they never call set_theme_mod() since the Customizer does it for them.
However, themes outside the WP repo can do whatever they want.
If you look at WP_Customize_Manager::add_setting() | Method | ClassicPress Documentation and see the type parameter in the $args, it can be ‘option’, but it defaults to ‘theme_mod’.
Also, a lot of themes written before the Customizer (or by people who don’t like the Customizer) use individual options and make their own Settings page. Having every theme present their options a different way on a different page was terrible and hard to review, and why the theme review team made the rules to only use Customizer and one array.
I think a button in the Customizer could be for deleting the current theme’s registered options or for loading the default values that they registered. I have code that does this for getting a parent theme’s values. It seems obvious that if it’s in the Customizer, it only affects those themes that use the Customizer. The original petition didn’t mention where it should be though.