Fix `DefaultStyles`

lib/src/widgets/default_styles.dart

@@ -269,7 +269,7 @@ class DefaultStyles {
             fontWeight: FontWeight.bold,
             decoration: TextDecoration.none,
           ),
-          const VerticalSpacing(8, 0),
+          const VerticalSpacing(6, 0),
           const VerticalSpacing(0, 0),
           null,
         ),
@@ -282,7 +282,7 @@ class DefaultStyles {
             fontWeight: FontWeight.bold,
             decoration: TextDecoration.none,
           ),
-          const VerticalSpacing(8, 0),
+          const VerticalSpacing(6, 0),
           const VerticalSpacing(0, 0),
           null,
         ),
@@ -295,7 +295,7 @@ class DefaultStyles {
             fontWeight: FontWeight.bold,
             decoration: TextDecoration.none,
           ),
-          const VerticalSpacing(8, 0),
+          const VerticalSpacing(4, 0),
           const VerticalSpacing(0, 0),
           null,
         ),
@@ -410,6 +410,9 @@ class DefaultStyles {
       h1: other.h1 ?? h1,
       h2: other.h2 ?? h2,
       h3: other.h3 ?? h3,
+      h4: other.h4 ?? h4,
+      h5: other.h5 ?? h5,
+      h6: other.h6 ?? h6,
       paragraph: other.paragraph ?? paragraph,
       bold: other.bold ?? bold,
       subscript: other.subscript ?? subscript,