element-styles.html.psm.scss

@import "globals";

custom-element {
  --custom-var: $primaryColour;
}

polymer_styles.rb

Rails.application.config.assets.configure do |env|
  # Add ".psm" extension to wrap a file in a Polymer style module
  PsmWrapper = ->(input) { 
    <<~DOC
      <dom-module id="#{input[:name]}">
        <template>
          <style>
            #{input[:data]}
          </style>
        </template>
      </dom-module>
    DOC
  }
  env.register_engine ".psm", PsmWrapper, mime_type: "text/html"
end