{
    "modes": {
        "sass": {
            "name": "Sass",
            "highlighter": "ace/mode/sass",
            "extensions": ["sass", "css.sass"]
        }
    }
}

{
    "modes": {
        "sass": {
            "name": "Sass",
            "highlighter": "ace/mode/sass",
            "extensions": ["sass", "css.sass", "scss"]
        }
    }
}

Include "scss" as an extension for Sass
Include "scss" as an extension for Sass

While some syntax highlighting might be slightly off due to the
differences between Sass and SCSS, it is certainly better than no
syntax highlighting at all.

I have tested the highlighting on a few .scss files, and the syntax
highlighting works quite well.
