It looks like there is a typo in widgets/_button.scss where the disabled buttons are not getting the color from the $color-disabled variable, but instead from the $color variable. So, setting a different button color when disabled does not get used - the default color gets used.

The offending code is on line 744

Lines 743-745:
Code:
            @if $color-disabled != $color {
                color: $color !important;
            }
should be:

Code:
            @if $color-disabled != $color {
                color: $color-disabled !important;
            }
Thanks,
Matt