use variable for background color in dark mode

Author: MilovanovM

djangoql/static/djangoql/css/completion.css

@@ -4,6 +4,7 @@
   border: solid 1px #ccc;
   border-radius: 4px;
   background: white;
+  background: var(--body-bg, white);
   min-width: 183px;
   font-size: 13px;
 }