Vertically center lonely h1 section headings

Change-Id: Id8b2be0184170c726bd81bd4a8a5886f9959aa76
diff --git a/inst/reveal.js-4.6.1/css/theme/source/ids.scss b/inst/reveal.js-4.6.1/css/theme/source/ids.scss
index 26dabc5..d4c2c44 100644
--- a/inst/reveal.js-4.6.1/css/theme/source/ids.scss
+++ b/inst/reveal.js-4.6.1/css/theme/source/ids.scss
@@ -226,6 +226,10 @@
   margin-top: -0.5em !important;
 }
 
+.reveal h1:last-child {
+  margin-top: 250px;
+}
+
 .title-frame h1,
 .title-frame h2 {
     margin: 0 0 0 0;