https://forum.mendix.com/questions/8174/InputRefSelector%20(refkit.mpk)
Did you try this? (found with forum search option)
If you use right-click on the element -> inspect, you are able to view the classes used by the reference selector. In case you use the default sass styling files from Mendix, take a look at the _dijit-widgets.scss.
Upgrade your reference styling over here. For example, to change the border when you are typing:
.dijitFocused {
border-color: #FF0000;
}
If you have any questions, please let me know =)