cylc gui: remove main window hardcoded colours

This addresses part of #885.

This improves the interaction of cylc gui with various alternative GTK
themes. This means that under themes such as HighContrastInverse,
(or any with light text foreground colours) the tree view looks much
more natural and is more readable. However, the formerly blue/white
column highlighting is now a rather drab grey row highlighting... are
we OK with this?

@hjoliver, please review.