dialog under high contrast theme
- Adjusted background for 'Composite' under high contrast theme

Change-Id: I28e66fd07a6c513f0926b8b4b8163e3aa1d6a519
Signed-off-by: Niraj Modi <niraj.modi@in.ibm.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/191947
(cherry picked from commit 363418b210cb53bee06e5b204db9260a392df720)
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/191856