Tweak the dark splash screen to be a bit darker.

packages/apputils-extension/style/splash.css

@@ -20,7 +20,7 @@
 }
 
 #jupyterlab-splash.dark {
-  background-color: var(--md-grey-800);
+  background-color: var(--md-grey-900);
 }
 
 .splash-fade {