update markdown h2 bottom-margin calculation

* calculation must use units; 'normal' value breaks calculation
* replace --header-line-height variable with line height unit (lh)
* introduce line heigh multiplier to reduce or increase line height

Author: dajrivera

src/themes/shared/_markdown.css

@@ -276,9 +276,11 @@
 
   h2 {
     --_spacing: 0.5em;
+    --_line-height-muliplier: 0.9;
 
     margin-bottom: calc(
-      var(--_spacing) + (var(--heading-line-height) - var(--font-size-xxl))
+      var(--_spacing) + (1lh * var(--_line-height-muliplier)) -
+        var(--font-size-xxl)
     );
     padding-bottom: var(--_spacing);
     border-bottom: 1px solid var(--border-color);