added variable for hero credit color

assets/sass/_theme/_configuration.sass

@@ -217,6 +217,7 @@ $hero-height: 300px !default
 $hero-height-desktop: 500px !default
 $hero-color: $color-text !default
 $hero-background-color: $color-background-alt !default
+$hero-credit-color: $color-text-alt !default
 
 // Breadcrumb
 $breadcrumb-color: $hero-color !default

assets/sass/_theme/_utils.sass

@@ -349,7 +349,7 @@ $space-unit: 4 !default
 @mixin collapsed-figcaption
     figcaption
         @include meta
-        color: $color-text-alt
+        color: $hero-credit-color
         position: absolute
         display: block
         left: 0