Avoid white as button-clear text color for .bar-light and .bar-stable

Currently, if you have a header bar with class="bar-stable" (or "bar-light") followed by a "button-clear" button, the text will be #fff. This means that button-clear text will be white on a bar-light or bar-stable bar, and thus unreadable.

This changes _bar.scss .bar-light and .bar-stable .button to @include button-clear a darker text color, currently $bar-light-text and $bar-stable-text respectively.

For consistency, perhaps either all @include button-clear should reference $bar-...-text, or the $fff should be changed to #444444 (or something similar) in _bar.scss