BREAKING CHANGES:
`$toolbar-md-button-color` no longer gets passed to the function that
sets the contrast color for toolbar buttons, but it can still be used
to set the default button color.
`$bar-button-md-color` was renamed to `$toolbar-md-button-color`
`$bar-button-md-border-radius` was renamed to
`$toolbar-md-button-border-radius`
references #6364