Fix dark theme completer text color.

packages/completer/style/index.css

@@ -13,6 +13,7 @@
 
 .jp-Completer {
   background: var(--jp-layout-color1);
+  color: var(--jp-content-font-color1);
   border: var(--jp-border-width) solid var(--jp-border-color1);
   box-shadow: 0px 1px 6px rgba(0, 0, 0, 0.2);
   list-style-type: none;