correct background color of quicksearch help footer

Test Plan:
- Open the Quick Search in slack (Command+K on os-x, Ctrl+K on Win)
- Enter some search term
- Verify that the entire dialog, including the footer (where the
  keyboard shortcuts are listed) is styled for the theme

scss/modules/modals/_quick-search.scss

@@ -4,7 +4,9 @@ ts-jumper {
     box-shadow: 0 1px 10px $color-shadow-dark;
   }
 
-  ts-jumper-help {
+  ts-jumper-help,
+  .p-jumper__help {
+    background: $color-base;
     color: $color-highlight;
   }