currently, `$gray-200` is practically imperceptible. switch to reusing `$dropdown-border-color`.
$dropdown-border-radius: $border-radius !default;
$dropdown-border-width: $border-width !default;
$dropdown-inner-border-radius: subtract($dropdown-border-radius, $dropdown-border-width) !default;
-$dropdown-divider-bg: $gray-200 !default;
+$dropdown-divider-bg: $dropdown-border-color !default;
$dropdown-divider-margin-y: $spacer / 2 !default;
$dropdown-box-shadow: $box-shadow !default;