Fixed an input field's background color in dark theme

Author: ollieh

src/skins/vector/css/matrix-react-sdk/views/dialogs/_ConfirmUserActionDialog.scss

@@ -37,6 +37,7 @@ limitations under the License.
     font-family: 'Open Sans', Arial, Helvetica, Sans-Serif;
     font-size: 14px;
     color: $primary-fg-color;
+    background-color: $primary-bg-color;
 
     border-radius: 3px;
     border: solid 1px $input-border-color;