Currently, You only add css for DE theme by add your css into directoryengine-child/style.css file.
This way only available for the desktop version(don’t use for the mobile version).
This is a child-theme for DE .This way allow you can edit the default css of mobile version by adding your css into child-theme/custom-mobile.css file.
Should add !important after you css to make it work better.
You can download here
Hope it helps!