Just having a look. The upshrink image is called in index.template.php -
// the upshrink image, right-floated
echo '
<img id="upshrink" src="', $settings['images_url'], '/upshrink.png" alt="*" title="', $txt['upshrink_description'], '" style="display: none;" />';
It's in Themes/Curve_Multi_Color/images/.
Main menu looks normal to me in IE11. Header image is overflowing right. Responsive curve mod will likely be a problem. It's designed to work only with default Curve, I can fix some of the glitches in Curve MC, but probably not all, especially with a lot of mods running. The best responsive theme I've worked with so far (I've only recently started messing with this) is mobile-desktop. Some mods have been updated to work with responsive themes, some haven't, you just have to try them. Best thing to do is to finalize whichever new theme you want to use, then we can try modding it for mobile.