OK, I tried changing the CSS sheet of the exported HTML5, and it worked. But have no idea if that would work when exporting to Android.
Tried executing this using the browser plug-in:
document.styleSheets[0].addRule('#sliderID', '-webkit-appearance: none');
document.styleSheets[0].addRule('#sliderID::-webkit-slider-thumb', '-webkit-appearance: none');
document.styleSheets[0].addRule('#sliderID::-webkit-slider-thumb', 'background: #FF0000');
It worked when I tried it in a sample html page. Didn't work in editor. As apk, when the code executes the slider disappears all together!
Ended up making my own slider! Pinning a circle sprite (with DragDrop) to a bar sprite (rope style).