Dialog buttons should use primary color

[touficbatache]

Bugs

What MDC-Web Version are you using?

0.15.0

What browser(s) is this bug affecting?

All browsers

What OS are you using?

Windows 7 / Android 7.0

What are the steps to reproduce the bug?

1. Go to http://material-components-web.appspot.com/dialog.html
2. Observe the component's behavior

What is the expected behavior?

The button text should use the primary color

What is the actual behavior?

The button text is black

Any other information you believe would be useful?

All the photos show a dialog with colored buttons: https://material.io/guidelines/components/dialogs.html

[lynnmercier]

Our http://material-components-web.appspot.com sets it's primary color to black. So this is working as intended.

--mdc-theme-primary: #212121;