Sorry, I am not able to reproduce your problem.
Do you have other css in your custom css file? Does the other css work?
For example if you add a text component to your DS App, and the following CSS into your custom css file: .sapUiTv {color: red;}
do you see the text with red color?
Do you run your app locally? If yes, is your custom css file in your repository folder of type: "Cascading Style Sheet Document (.css)"?
Did you attach/link your file to your DS app as: name_customcss.css - with dot css at the end?
This is no longer necesssary, at least not in DS v15.0.6, but you can clear browser cache and reload page again to force the browser to load the latest version of the custom css file.
Regards,
David