fix: add missing CSS Variable

Author: mrholek

scss/_dropdown.scss

@@ -213,6 +213,7 @@
   --#{$variable-prefix}dropdown-border-color: #{$dropdown-dark-border-color};
   --#{$variable-prefix}dropdown-link-color: #{$dropdown-dark-link-color};
   --#{$variable-prefix}dropdown-link-hover-color: #{$dropdown-dark-link-hover-color};
+  --#{$variable-prefix}dropdown-link-active-color: #{$dropdown-dark-link-active-color};
   --#{$variable-prefix}dropdown-link-disabled-color: #{$dropdown-dark-link-disabled-color};
   --#{$variable-prefix}dropdown-divider-bg: #{$dropdown-dark-divider-bg};
   --#{$variable-prefix}dropdown-header-color: #{$dropdown-dark-header-color};