Use the mode instead of the sass variable to determine which color the
buttons in a toolbar should be based on the background color. Added
error messages when anything other than a color is passed and more
comments.
BREAKING CHANGES: ios mode `$toolbar-ios-button-color` now has a
default value of `color-contrast($colors-ios, $toolbar-ios-background,
ios)` which will evaluate to the primary color for light background
toolbars and white for dark background toolbars.
fixes#6364
BREAKING CHANGES:
`$bar-button-wp-color` was renamed to `$toolbar-wp-button-color`
`$bar-button-wp-border-radius` was renamed to
`$toolbar-wp-button-border-radius`
Added `$toolbar-wp-title-text-color` for better control of the title
color
Removed `$toolbar-wp-button-color` from the default themes
references #6364
