Adding a style in ACP and setting it as default replaces users' style

When adding a style in ACP there is a radio button to set it as the default style.

When it is set to "yes", adding the style replaces all users' style with the new style.

Users' style selection should only be overridden if explicitly requested by the admin, i.e., they were given the option to "replace users' styles" and chose to do so.