If you customize your site, you can return it to the out-of-the-box defaults if necessary. In this case, you would completely wipe a site's theme back to the out-of-the-box installation - discarding any customizations that a site administrator, group owner, blog author, or individual might have made to any pages on the site.
To revert a site to out-of-the-box defaults:
Set up a new site.
Export all of the defaults for the themes (which you should keep for each version anyway).
Restore defaults on all themes on your site.
Delete all content in the cs_ThemeConfiguration table and cs_ContentFragmentPages table (where ThemeContextId <>-1) on your site.