BoxesMiniGraph: Replace hardcoded colors by theme colors
BoxesMiniGraph: Replace hardcoded colors by theme colors

The BoxesMiniGraph will now invert colors with the theme if needed.

This is a minor visual change while it allows Boxes to adapt to theme
changes and especially makes themes like the high contrast accessibility
theme work.

https://bugzilla.gnome.org/show_bug.cgi?id=731952

vbenes/gnome-boxes,vbenes/gnome-boxes