Have $form-check-input-border's default derive from ($black instead of hard-coded black)

Author: cpsievert

scss/_variables.scss

@@ -738,7 +738,7 @@ $form-check-transition:                   null !default;
 $form-check-input-active-filter:          brightness(90%) !default;
 
 $form-check-input-bg:                     $input-bg !default;
-$form-check-input-border:                 1px solid rgba(0, 0, 0, .25) !default;
+$form-check-input-border:                 1px solid rgba($black, .25) !default;
 $form-check-input-border-radius:          .25em !default;
 $form-check-radio-border-radius:          50% !default;
 $form-check-input-focus-border:           $input-focus-border-color !default;