With Hugo 0.46 you should be able to use resources for that.
Example of SCSS
{{ with resources.Get "scss/styles.scss" | toCSS | minify }}
<style>{{ .Content | safeCSS }}</style>
{{ end }}
or CSS
{{ with resources.Get "css/styles.css" | minify }}
<style>{{ .Content | safeCSS }}</style>
{{ end }}
Note that resource files needs to be located under /assets directory (not /static)