Check out the documentation on Mendix and SASS if you want to go down that route: there are a few things you need to know and do in order for this to work :)
In order for your changes to take effect you need to recompile (with Koala, Gulp or Calypse for instance). Furthermore, you should not add the changes to the _custom.scss file, this is where the changes are compiled to from other files. See the documentation for more information on how this works exactly.
Good luck, and be sure to post questions if you get stuck!