The following blog will help you to achieve it
I think your scss files doesn’t sync or compiled with project properly.
Try with project→ synchronize project directory in modeler. If it doesn’t work then you need to compile scss file manually with the help of calypso app.
Hope this help
You can check the sources used by your application, using the console of the browser (f12). See for instance the example of forum.mendix.com:
by checking this, you can see if your custom scss is included. And if not, you should clear cache of the browser, clean deployment directory.