Make header margin an overridable setting.

css/theme/template/settings.scss adds the $headingMargin variable, set to `0 0 20px 0`.
css/theme/template/theme.scss uses $headingMargin to set margin on headers.

css/theme/template/settings.scss

@@ -10,6 +10,7 @@ $mainFontSize: 36px;
 $mainColor: #eee;
 
 // Headings
+$headingMargin: 0 0 20px 0;
 $headingFont: 'League Gothic', Impact, sans-serif;
 $headingColor: #eee;
 $headingLineHeight: 0.9em;

css/theme/template/theme.scss

@@ -33,7 +33,7 @@ body {
 .reveal h4,
 .reveal h5,
 .reveal h6 {
-	margin: 0 0 20px 0;
+	margin: $headingMargin;
 	color: $headingColor;
 
 	font-family: $headingFont;