Apparently there was a problem with your TikiWiki installation not related to the theme. Normally, changing the theme doesn't result in you being locked out of the site. (The theme only controls page appearance, not underlying functionality.)
I see from visiting the url on your screenshot that trying to see the site causes a database error. I suggest that you first of all make sure all the TikiWiki files are installed and then use tiki-install.php to update your database (this will add anything that's missing, but won't affect your site content). When at least the TikiWiki installation is intact (that is, some kind of page displays as expected), then the log-in and theme-related problems can be addressed.