make LinearColorBar themeable

Move colors to resources and use them instead.

Change-Id: I923e5daa219f302dc24e62faab666483109e6620
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
Themes: data usage chart: move color values to resource file

This will allow themes to specify custom colors for
the data usage chart.

Change-Id: I17669348e8ac6dce63100604c1044f9de78056c2
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>