When a style is marked as unselectable in XenForo, the user information in the user table still contains the unselectable style ID for every user that used it.
This add-on corrects that by placing the site's default style ID into the user information for those users.
This can also be used to force everyone to use a new style. To do that, mark the new style as the default style and then mark every other style as unselectable. After than is completed, all users will be using the new style. Then if you wish, you can then go back and make the other styles selectable again. Users will then be able to change styles to the one they want to use after seeing the new style.
NOTE: If using this with my Style Statistics add-on, you do not need to reset the statistics when a style is marked as unselectable. The statistics will auto-correct as each user comes online.
This DOES NOT change the style admins are using because admins can use any style that's available.