I would suggest never to work in the css files. They will be overwritten whenever someone adds changes to the scss files and compiles them.
In 8.2.2, I would add my custom stuff in a .scss file somewhere in theme\styles\web\sass\app and import it into the custom.scss file. You can use Gulp, Koala or Calypso to compile it.
I recommend setting up your UI framework using Koala or Calypso, and then editing your custom.scss file.
Here is a link to get you setup with Koala
and then edit your custom.scss file at:
Hope this helps!
"theme\styles\sass\custom" moved to