csso cannot parse custom properties

[matype]

input.css

:root {
  --mainColor: #fb4c42;
}

and run the following command:

$ csso input.css

It occurs this error.

Parse error /Users/a13970/7gogo/7gogo-web/dist/css/app.css: Identifier is expected
    1 |:root {
    2 |  --mainColor: #fb4c42;

I like csso because it is faster than the other css minify tools e.g. cssnano :)

I wish your help.