You can do in the following way :-
i) Create the style you want to set for the disabled combobox(similar to enabled text box).
Let say the class name is '
.disabledNewStyle'.
ii) Get the disabled combobox in jQuery and add the new class to it.
Let say the disabled combobox ID is '
cmbDisabled'.
Now you can do like below :-
$( "#cmbDisabled:disabled" ).addClass('disabledNewStyle');
// also you can use '.css' method to add styles directly and make it '!important' to override previous styles.
$( "#cmbDisabled:disabled" ).css("color", "black !important");
$( "#cmbDisabled:disabled" ).css("border", "black !important");
Hope this will be of help, at least give you a hint to go in correct way.