My quick fix for this issue:
In the file wp-content/plugins/nextgen-gallery/widgets/widgets.php, line 381&382:
$before_title .= '<h2 class="widgettitle entry-title">';
$after_title = '</h2>';
I changed to the following, to match the css from the theme:
$before_title .= '<div class="widget-title entry-title"><h3>';
$after_title = '</h3>';
The header now matches up with the rest of the sidebar font changes.
