CSS styling regression on PopupControls due to hierarchy change in RT-35175

Because of a change in RT-35175 David and I have identified visual regressions in controls based on PopupControl, such as the ComboBox.

Refer to the before / after screenshots - note that in the before picture (which was taken before the bad changeset was pushed to the repo) there is no alternate row highlighting and a hover highlight. This is what is expected. After the bad changeset there is alternate row highlighting and no hover highlight.