refs #19352 [fid_bbi] fix heading styles if <h1> is missing

For templates with missing <h1>, style the first <h2> as <h1> and subsequent headings as one level higher.

themes/fid_bbi/scss/blocks/main.scss

@@ -15,4 +15,17 @@
   > div:not([class]) {
     @include paragraph;
   }
+
+  // For templates with missing <h1>
+  > h2:first-child {
+    @include heading(1);
+
+    ~ h3 {
+      @include heading(2);
+    }
+
+    ~ h4 {
+      @include heading(3);
+    }
+  }
 }