Improve syntax highlighting colors for dark theme in MANIFEST.MF

Changing the color to the same as for classes in the Java editor and
removing the bold setting.

Fixes #1503

ui/org.eclipse.pde.ui/css/e4-dark_pde_prefstyle.css

@@ -23,9 +23,9 @@ IEclipsePreferences#org-eclipse-pde-ui:org-eclipse-pde-ui {
 		'editor.color.header_assignment=103,207,241'
 		'editor.color.header_assignment_bold=true'
 		'editor.color.header_attributes=140,210,20'
-		'editor.color.header_key=221,40,103'
-		'editor.color.header_key_bold=true'
-		'editor.color.header_osgi=38,139,210'
+		'editor.color.header_key=18,144,195'
+		'editor.color.header_key_bold=false'
+		'editor.color.header_osgi=18,144,195'
 		'editor.color.header_value=23,198,163'
 		'editor.color.header_value_italic=true'
 		'editor.color.instr=221,40,103'