Add a new color --text-fg-on-accent-bright

For allowing the designer to select what color text should be when on
a background of --accent-color-bright.

Note: The value for this color happens to match the color of
previously-defined values currently, but that's just me throwing
programmer-art colors in place. That's why we don't reference an
existing variable in the value here, even though the color is
currently identical to a previous one, (we wouldn't want a designer to
think they are necessarily linked).