Change backgroud color for diff context control background under dark theme
Change backgroud color for diff context control background under dark theme

To bring more contrast

Bug: Issue 11786
Change-Id: I280360397b690697dabb9cbe7d84f9a1758344aa

GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit