Sorry, but we have no super-powers to guess what is in
myCustom.css.
(Comic-Con is in town here in San Diego, and I doubt I could even find a super-hero there with those particular powers. Lots of people running around *dressed* as super-heros, but, alas, no actual CSS-guessing superheros...)
https://forum.jquery.com/topic/how-to-ask-for-jquery-mobile-helpIt's best to give us a complete online example we can try and examine.
I'd imagine the problem might have to do with specificity.
jQuery Mobile doesn't (and can't) make CSS work any differently than it works with any other platform or with no platform at all.
It's best to use a tool like Web Inspector or Firebug so that you can inspect the CSS applied to a given element and see exactly what rules have been applied to a given element. Then you can work-out the selectors needed for overrides. Again, nothing special about jQuery Mobile in this regard - it's just CSS.