Apply opacity to background-color on PreferencesBox

opacity on an element applies to all child elements and cannot be
removed. This is a bit awkward when the button or text of the
PreferencesBox is white, since you can see the underneath
content. Instead just apply opacity to the background color, so
everything else in the box is no opaque.

Change-Id: Ie37cd16770030e4aca2067d37d2f6d82ac25ddfa

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/PreferencesBox.ui.xml

@@ -28,7 +28,7 @@ limitations under the License.
     @external .gwt-ToggleButton-down-disabled;
 
     .dialog {
-      background: #000000 none repeat scroll 0 50%;
+      background: rgba(0, 0, 0, 0.85) none repeat scroll 0 50%;
       color: #ffffff;
       font-family: arial,sans-serif;
       font-weight: bold;
@@ -36,7 +36,6 @@ limitations under the License.
       text-align: left;
       text-shadow: 1px 1px 7px #000000;
       min-width: 300px;
-      opacity: 0.90;
       z-index: 200;
       -webkit-border-radius: 10px;
       -moz-border-radius: 10px;