How do I get Contao to recognize my .scss file

I'm trying to learn Sass and use it with Contao 3.3.3. I've set up a .scss file and have added it to my page layout in the additional style sheets field, but it's not working. I'm probably missing something. Does anyone know how to make this work?

I think my overall concept of how it should work was off. I was treating the .scss file like the variables in the Contao's css system. For example, i added $blue: #296dc1; to my .scss file, assuming that everywhere I have $blue in the Contao css manager it would apply it. But that was not the case. I have to add the selectors to my .scss as well. It's not using the .scss file to inform the other css files, it's just adding the outputted css file to the mix.