use-navbar-inverse-border

In `.navbar-default`,  `.navbar-collapse, .navbar-form` is `border-color: $navbar-default-border;` but in `.navbar-inverse'`, `.navbar-collapse, .navbar-form` is hard-coded like `border-color: darken($navbar-inverse-bg, 7%);`.

Is this has some means? I think `border-color: $navbar-inverse-border;` is better though `$navbar-inverse-border` is different from `darken($navbar-inverse-bg, 7%)`.