_sass: Set card header font size to text-gamma

_sass/custom/announcement.scss

@@ -3,6 +3,7 @@
 }
 
 .announcement-header {
+  @extend .text-gamma;
 }
 
 .announcement-body {

_sass/custom/module.scss

@@ -101,6 +101,7 @@
 }
 
 .module-header {
+  @extend .text-gamma;
 }
 
 .module-body {