style: markmap style on slide mode

Signed-off-by: Raccoon <raccoon@hackmd.io>

Author: a60814billy

public/css/slide.css

@@ -344,3 +344,7 @@ html, body {
 .print-pdf .footer {
     display: none;
 }
+
+.markmap-container {
+  background: #f7f7f7;
+}