I have updated my own disabled styling css rule to include the opacity property. I believe that this will accomplish what you're looking to do.
color: #969696 !important;
I have never used Nintex Forms for Office 365.
Are there other css rules inside of that panel? If so, then yes, just paste the above rule into that panel.
If it doesn't accept css rules outright, then I imagine that it would accept a url pointing to a valid css file.