Hey Michael

The datepicker is coming from a 3rd party HTML5 library, and I'm not sure that you can modify it that easily.

I'll forward the question to one of our developers, and if he knows of a way I'll let you know.

Hi again Michael.

You first need to find those inputs in inspect element:

Then, in Source, find the file wdt.funcs.js on line 160 add a breakpoint:

And then click on one of the inputs in the date range. The debugger will stop there, and after that you need to switch to the Element tab, and you will see the element of that datepicker:

One of the CSS you can use, for the background of the header (for example) is: 

.wpdt-c .wpDataTablesWrapper .bootstrap-datetimepicker-widget.wdt-datetimepicker-modal thead th{
 background-color: red !important;
}

Or the background of a selected date:

.wpdt-c .wdt-datetimepicker-modal .datepicker table td.active:before {
 background-color: red !important;
}

I hope that helps.

You're welcome, Michael

If you have any further questions or issues, please feel free to open a new ticket, and we'll gladly help.