HC: Fix fgcolor of backdrop:disabled spinbutton +-

We shouldn't give the icons here the same fg colour as the bg... which
makes them disappear and the buttons look like meaningless flat squares.

Fix by just using the same colour the same as foreground disabled. Note:
insensitive_fg_color is more prominent than !disabled, so clearly wrong.
2 jobs for wip/dboles/spinbutton-button-backdrop-disabled-3 in 8 minutes and 19 seconds (queued for 3 seconds)