When viewing available themes, how to change prevew to link to a demo

I've always felt that live preview of installed themes is pretty much useless, since so many themes depend on specific types of content and add-ons to achieve the look you see in the thumbnail. In fact, many themes look just plain broken. Thus, I would like to change the link of the Live Preview button to a Demo button instead. I would like to do this without hacking wordpress core files - namely /wp-admin/themes.php - but I'm guessing a core hack would be necessary to achieve this.

I found the link at line 325 of wp version 4.1.1 which outputs the Live Preview button as follows:

<a class="button button-primary load-customize hide-if-no-customize" href="{{{ data.actions.customize }}}"><?php _e( 'Live Preview' ); ?></a>

I would like to change it to:

<a class="button button-primary load-customize hide-if-no-customize" href="http://demo.example.com/<?php respective_theme(); ?>"><?php _e( 'View Demo' ); ?></a>

Where I'm stuck is how to get the directory, such as 'twentyfifteen' for the Twenty Fifteen theme, for example. Any ideas how to retrieve the respective theme directory?

Also, is there a way to do this without hacking the core file?

Thanks!