Need place holder theme that still shows after login.

Placeholder themes are obviously shown over the site's regular theme. However I would like to have it as the real theme. Is this easy editing like is it a child theme or something? Here's script to the theme I'm using.

See what I'm doing is blocking users from using a specific browser , so with theme to browser plugin , I've selected a placeholder theme for that browser , the problem is if they login on this browser they still get the underlying theme with this work around because placeholder themes don't take over everything.

My alternative to having a pretty place holder do the job would be to mess up a regular theme so that they don't get anywhere on the site even after logging in through that browser.