Change sort button colour

Change from grey to blue to math rest of theme
Sort button icon and colour

Switch to Fontawesome icon, solid bkg colour and delete old bkg image.
[2.3] Grid header bar colours
SASS colour variables

Switch hardcoded colours to SASS variables
Merge branch 'sortButtonColour' into gridHeaderBar
SASS variables

Move rest of colours to SASS variables
Merge remote-tracking branch 'upstream/pr/11673' into pr/11673

* upstream/pr/11673:
  SASS variables
  SASS colour variables
  [2.3] Grid header bar colours
  Sort button icon and colour
  Change sort button colour