If you have installed a new WordPress theme and then activate it and get a blank screen admin disaster - you just can not access the Admin login or any admin page - your actual website content might work but the style might be missing or you have other issues but thats no good to you or your visitors - then follow these instructions to be able to access the WordPress Admin.
Using your Cpanel or FTP (File Transfer Protocol software like FileZilla) go into your site, find the "wp-content" file then go to the "themes" file and delete the new WordPress theme you installed that created the blank screen Admin issue.
Try to access your WordPress Admin pasting in the url and not any other method. So create the url yoursite.com/wp-admin or /wp-login.php and then paste and go.
This should allow you to login back in and have access to the WP Admin page. Then you need to click on the Appearance > Theme page and you will get a message like "The active theme is broken. Reverting to the default theme." WordPress will automatically do it for you and go back to your default theme that you were using.