It's a bunch of .js files stored in
$SPLUNK_HOME/share/splunk/search_mrsparkle/exposed/js, I've done some changes myself for showcases - so yeah, it is possible.
However, any changes made there will certainly not survive an upgrade and won't make any friends in support.
Instead, have you considered using SplunkJS/Webframework to build your own search page? That would live in your local space, survive upgrades, and not mess with built-in features. Here's a pointer on where you might get started: http://dev.splunk.com/view/SP-CAAAET9