finally found the CSS code to make the selection of listboxes better readable

Putting it in default.css instead of my own userChrome.css, because also others have mentioned
the hard readability.

chrome/content/vimperator/default.css

@@ -143,6 +143,15 @@ fieldset.paypal {
     overflow: -moz-scrollbars-none;
 }
 
+/* the selected item in listboxes is hardly readable without this */
+#vimperator-completion > listitem[selected="true"],
+#vimperator-bufferwindow > listitem[selected="true"],
+#vimperator-previewwindow > listitem[selected="true"]
+{
+   background-color: #C9DFFF !important; 
+}
+
+
 #vimperator-statusline > label {
     padding: 0px 0px 0px 8px;
 }