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:
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Set up a new site.
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Export all of the defaults for the themes (which you should keep for each version anyway).
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Restore defaults on all themes on your site.
or
Delete all content in the cs_ThemeConfiguration table and cs_ContentFragmentPages table (where ThemeContextId <>-1) on your site.