Adding a variable to handle the change of a modals border-radius. (#25691)

Author: akolinski

scss/_modal.scss

@@ -67,7 +67,7 @@
   background-color: $modal-content-bg;
   background-clip: padding-box;
   border: $modal-content-border-width solid $modal-content-border-color;
-  @include border-radius($border-radius-lg);
+  @include border-radius($modal-content-border-radius);
   @include box-shadow($modal-content-box-shadow-xs);
   // Remove focus outline from opened modal
   outline: 0;
@@ -96,7 +96,7 @@
   justify-content: space-between; // Put modal header elements (title and dismiss) on opposite ends
   padding: $modal-header-padding;
   border-bottom: $modal-header-border-width solid $modal-header-border-color;
-  @include border-top-radius($border-radius-lg);
+  @include border-top-radius($modal-content-border-radius);
 
   .close {
     padding: $modal-header-padding;

scss/_variables.scss

@@ -769,6 +769,7 @@ $modal-title-line-height:           $line-height-base !default;
 $modal-content-bg:               $white !default;
 $modal-content-border-color:     rgba($black, .2) !default;
 $modal-content-border-width:     $border-width !default;
+$modal-content-border-radius:    $border-radius-lg !default;
 $modal-content-box-shadow-xs:    0 .25rem .5rem rgba($black, .5) !default;
 $modal-content-box-shadow-sm-up: 0 .5rem 1rem rgba($black, .5) !default;