You can also just override css in your own local css. That is, embedded or in a file that you import into your page, add the css rule:
.ui-state-disabled { background-image: none; opacity: 1; }
Where, in this case, I redefined the opacity value.
This can cause problems if you really want to use this class elsewhere, in which case programmatically adding / removing a class to / from selected elements (as Alphonso suggests) is probably the best way.
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