Trouble with theme enabling

Hi. I'm new in front-end development . Could you please help with my issue, here is the problem:
I read a few times this guide: https://akveo.github.io/nebular/#/docs/guides/enabling-theme-system
but it dosen't help me because i have not a Angular CLI project (asp.net core + angular4 template). I have installed default prebuild styles and my themes have changed. But i still can't use something like // . Please show me the way how can i fix it. Thanks!