Sync custom select focus state with custom control focus state

scss/_variables.scss

@@ -591,7 +591,7 @@ $custom-select-box-shadow:          inset 0 1px 2px rgba($black, .075) !default;
 
 $custom-select-focus-border-color:  $input-focus-border-color !default;
 $custom-select-focus-width:         $input-focus-width !default;
-$custom-select-focus-box-shadow:    0 0 0 $custom-select-focus-width rgba($custom-select-focus-border-color, .5) !default;
+$custom-select-focus-box-shadow:    0 0 0 $custom-select-focus-width $input-btn-focus-color !default;
 
 $custom-select-padding-y-sm:        $input-padding-y-sm !default;
 $custom-select-padding-x-sm:        $input-padding-x-sm !default;