Add news item for automatic theme

assets/javascripts/news.json

@@ -1,4 +1,8 @@
 [
+  [
+    "2020-01-13",
+    "New “Automatic” theme: match your browser or system dark mode setting. <a href=\"/settings\">Enable it in preferences</a>."
+  ],
   [
     "2020-01-13",
     "New documentation: <a href=\"/gnuplot/\">Gnuplot</a>"