Add default fallback for title font weight

This approach ensures we have a common default (bold) across all
themes that don't choose to set a custom font-weight

Author: maureenlholland

Diff for: assets/sass/protocol/base/elements/_titles.scss

@@ -12,9 +12,10 @@ h3,
 h4,
 h5,
 h6 {
+    $default-font-weight: bold;
     font-family: $title-font-family;
     color: $title-text-color;
-    font-weight: bold;
+    font-weight: $default-font-weight;
     margin: 0 0 0.5em;
 
     .mzp-t-dark & {
@@ -24,7 +25,7 @@ h6 {
     @supports (--css: variables) {
         font-family: var(--title-font-family);
         font-variant-ligatures: var(--title-font-ligatures);
-        font-weight: var(--title-font-weight);
+        font-weight: var(--title-font-weight, #{$default-font-weight});
         letter-spacing: var(--title-letter-spacing);
         color: var(--title-text-color);