fix: add explicit h5 heading styles to maintain size hierarchy (#381)

* fix: add explicit h5 heading styles to maintain size hierarchy

* style: use 1rem instead of 1.0rem for consistency

Author: mmcky

src/quantecon_book_theme/assets/styles/_base.scss

@@ -147,6 +147,13 @@ h4 {
   color: #000;
 }
 
+h5 {
+  font-size: 1rem;
+  font-family: "Source Sans Pro", sans-serif;
+  font-weight: 900;
+  color: #000;
+}
+
 // Emphasis styling - replace italics with color and semi-bold weight
 em {
   font-style: normal;