- make sure you are in the correct project directory. In MendixStudioPro click project->project directory, guess you've done that.
- Make sure the _custom.scss has been compiled. ‘sass --watch sass:css’ will do the trick.
- Make sure the compiled CSS is deployed to your deployment directory. Pressing F5 in MendixStudioPro will do that for you
Another option is to use gulp of calypso, which will do 2 and 3 automatically.
To check in the browser if the new css is used, use the inspector (F12), tab ‘Sources’ and check http://localhost:8080/styles/web/css/main.css?637147240541340220. It is your css on one line, but with Find you can look for your latest addition.