add a fotgotten color-variable

src/client/styles/scss/_override-bootstrap-variables.scss

@@ -63,6 +63,7 @@ $input-placeholder-color: $gray-500;
 
 $custom-control-indicator-border-color: $gray-400;
 $custom-control-label-disabled-color: $gray-500;
+$custom-select-disabled-color: $gray-500;
 $custom-range-thumb-disabled-bg: $gray-400;
 
 //== Navs