labwc-theme(5): describe color alpha value

labwc-theme(5): describe color alpha value

...which has been supported for a long time but not been reflected in the
man page.  Colors can be parsed as `#rrggbb aaa` with aaa representing a
decimal alpha value. This could be used to hide a button, for example:

    window.active.button.menu.unpressed.image.color: #000000 0
    window.inactive.button.menu.unpressed.image.color: #000000 0