Reduce vertical margins for h4 - h6

Change-Id: I7fdad39b8273f13d4fc445bd469bcd204873d47e
diff --git a/inst/rmarkdown/templates/posterdown_ids/resources/template.html b/inst/rmarkdown/templates/posterdown_ids/resources/template.html
index 6732db0..8edadfc 100644
--- a/inst/rmarkdown/templates/posterdown_ids/resources/template.html
+++ b/inst/rmarkdown/templates/posterdown_ids/resources/template.html
@@ -270,6 +270,11 @@
 margin-block-end: 0.25em;
 }
 
+.poster_body h4, h5, h6 {
+margin-block-start: 0.5em;
+margin-block-end: 0.25em;
+}
+
 .span {
 width: 200%;
 }