Some theme engines, such as ClearLooks, appear to use different color values when painting the progress bars. Because of this, the bars always appear in the default theme color, regardless of what's set in the settings dialog. I'll attach a patch to fix this.
Created attachment 1383 systemload-fix-bar-colors.diff Patch also uses the gtk_widget_modify_bg() and gtk_widget_modify_base() functions instead of the old GtkRcStyle code, for simplicity.
Xubuntu bug for this : https://bugs.launchpad.net/xfce/+bug/67945
Lionel (mrpouit) confirmed this patch works.
Patch applied, and pushed to the master branch. Thanks for the contribution.
*** Bug 6745 has been marked as a duplicate of this bug. ***