Docs styles corrupted in Firefox

[web-padawan]

Currently the default-theme is applied using <custom-style>, but this does not work in polyfilled browsers (e. g. Firefox), most likely because of selector in this line.

The simplest solution is to add a Polymer element and then import style module inside of that element, like iron-component-page does.

[web-padawan]

The screenshot below demonstrates how it looks like now:

As you can see, the custom-style does not get processed correctly: