It looks like the only way to make a mass change is via the command line. The script /usr/local/cpanel/scripts/modify_accounts will allow you to change the style of your users. Here is an example for all users:
/usr/local/cpanel/scripts/modify_accounts --all-users --theme=paper_lantern --style=basic
Running the script with --help will show all options for the script.
Once you make that change you likely want to remove the change style privilege that 24x7server mentioned.