It not recommended to add styling the main CSS. This will give you trouble upgrading to newer version of Atlas themes in the future.
If you want to add a new CSS file, you have to reference this in <projectfolder>/theme/settings.json
Include here the relative path, for example
"cssFiles": [ "styles/web/css/main.css", "styles/web/css/my-custom-file.css" ],
And work with SASS you can use Calyso https://docs.mendix.com/howto/front-end/calypso#1-introduction for compiling.
The best option is to use sass. See your theme folder /theme/styles/web/sass
See this tutorial: https://docs.mendix.com/howto/front-end/style-with-gulp-and-sass#1-introduction