try also gulp + mendix https://docs.mendix.com/howto/ux/style-with-gulp-and-sass and https://github.com/mendix/ux-theming
You could try setting up Koala, I found it a lot easier to get working. Once you install it, all you have to do is drag the styles folder (from the themes folder) into the application and it will set everything up for you. Then any time you save a sass file, Koala will recognize that and compile the css files.
Here is a guide to get it setup
Here is where to download koala
Hope this helps!
Yeah, this actually is not an error per se...
It just works. But the item it finds in your folder, is not an scss file. But in the subfolders, it finds the scss files.
Save an scss file, and more green rows will appear.