fix: safari has a slight ui difference using lh and em units instead of variables

Author: dajrivera

src/themes/shared/_markdown.css

@@ -278,7 +278,8 @@
   h2 {
     --_spacing: 0.5em;
 
-    margin-bottom: calc(var(--_spacing) + (1lh - 1em));
+    margin-bottom: var(--_spacing) +
+      (var(--heading-line-height) - var(--font-size-xxl));
     padding-bottom: var(--_spacing);
     border-bottom: 1px solid var(--border-color);
     font-size: var(--font-size-xxl);