Change the background color of material website to make it more accessible

[amcdnl]

Reference: angular/components#9339

Per @jelbourn:

It does look like many of the syntax highlighting colors don't meet W3C contrast standards

[amcdnl]

@jelbourn - We just changed these styles per #301 to reflect https://developers.google.com/web/fundamentals/architecture/app-shell.

I do agree the methods and table look bad. I'll look into those.