Neilo,
You simply add your chose class to the date field, there is no need to add anything to the data assist css.
The UI script file you posted is 213kb which is approx. 3 times the size of the actual jQuery file plus another 33kb for the CSS file, this is one of things I don't like about jQueryUI but as with all these things it is just my opinion and in the long run you should use what you find easiest to implement.