Inherit host control colors in content proposal popups

Content proposal popups now fall back to the colors of the control they
are attached to when the JFace CONTENT_ASSIST_* color preferences are
unset. RCP applications that style their controls through SWT API,
without configuring these JFace preferences, get matching popups for
free. It also fixes the white completion popup in the dark-themed
workspace selection dialog, which is shown before the workbench theme is
applied.

Author: vogella

bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ContentProposalAdapter.java

@@ -594,14 +594,18 @@ boolean hasFocus() {
 
 		@Override
 		protected Color getForeground() {
-			return JFaceResources.getColorRegistry().get(
+			Color foreground = JFaceResources.getColorRegistry().get(
 					JFacePreferences.CONTENT_ASSIST_FOREGROUND_COLOR);
+			// Fall back to the host control's color so the popup matches its
+			// surroundings when no content assist color is configured.
+			return foreground != null ? foreground : control.getForeground();
 		}
 
 		@Override
 		protected Color getBackground() {
-			return JFaceResources.getColorRegistry().get(
+			Color background = JFaceResources.getColorRegistry().get(
 					JFacePreferences.CONTENT_ASSIST_BACKGROUND_COLOR);
+			return background != null ? background : control.getBackground();
 		}
 
 		/*