UI: Markup: Slight highlight on strong and headings

Slight highlight on strong and heading elements, to improve
readability.

On dark themes they display slightly lighter. On light themes there
isn't much of a difference (they're already pure black).

templates/custom/header.tmpl

@@ -45,6 +45,13 @@
     color: var(--color-text-light-3);
   }
 
+  /* Markup. */
+  /* Slight highlight on strong and heading elements, to improve readability. */
+  .markup strong,
+  .markup h1, .markup h2, .markup h3, .markup h4, .markup h5, .markup h6 {
+    color: var(--color-text-dark);
+  }
+
   /* Homepage. */
   /* By default Gitea colors links green */
   .home a {