If you need to support Internet Explorer 11 and/or Edge 14, you need to apply styling in a certain way, because the CSS custom properties are applied using the ShadyCSS shim which has limitations.
If you wish to import the global theme styles, you need to do that in the root scope of your HTML. If you place the <custom-style> element inside an HTML import, it doesn’t work in IE11 or Edge14.
Also, you need to provide at least one white space character inside the <style> element.
In order to apply the dark theme colors, you need to introduce a new CSS scope in the form of a shadow root, as a custom element. The ShadyCSS shim only polyfills custom property values on shadow root boundaries.
The easiest way to do so is to create a new Polymer element, and include the “material-dark” style module inside its template:
dark-styles.html