Relates to #14699.

@import "hugo:vars";
@import "hugo:vars/mobile" (max-width: 650px);

And then:

{{ with resources.Get "sass/main.css" }}
  {{ $opts := dict
    "vars" (dict 
         "primary-color" "blue"
          "mobile" (dict "somemobilevar" "foo)
     ))
  }}
  {{ with . | css.Build $opts }}
      <link rel="stylesheet" href="{{ .RelPermalink }}">
  {{ end }}
{{ end }}

• The above should be trivial to implement for css.Build
• I'm not sure about css.Sass (or: it would be trivial to implement, but I'm not sure Dart Sass' import supports the relevant syntax), but we should try to make this work for both css.Build and css.Sass (but we don't spend time on the libsass transpiler, which is deprecated).

Relevant documentation:

• https://gohugo.io/functions/css/build/#vars
• https://gohugo.io/functions/css/sass/#vars