Fix style deprecation warning

[adi130987]

[Deprecation] Styling master document from stylesheets defined in HTML Imports is deprecated, and is planned to be removed in M65, around March 2018. Please refer to https://goo.gl/EGXzpw for possible migration paths.

We need to fix this style deprecation until March.

[adi130987]

@daniel-tabarcea I think you can test this issue on Chrome Canary, maybe Chrome dev.
We need to make sure we won't have issues because of this deprecation.

[daniel-tabarcea]

After some research I found out that this should not be an issue at all, because Polymer hoists CSS from external stylesheets to the main page, but the browser performs these checks before that can happen and it still displays the warning. I've tested the PMP app on the development build of Chromium 65 and there were no styling issues.
As for preventing the error from showing, we are waiting for a change to polymer that will allow this, the issue is 5017 in the polymer repo