feat(style): add @modal-header-border-color-split variable (#2179)

* add @modal-footer-border-color-split to modal.less

* add @modal-header-border-color-split to default.less

components/modal/style/modal.less

@@ -83,7 +83,7 @@
     padding: 16px 24px;
     color: @text-color;
     background: @modal-header-bg;
-    border-bottom: @border-width-base @border-style-base @border-color-split;
+    border-bottom: @border-width-base @border-style-base @modal-header-border-color-split;
     border-radius: @border-radius-base @border-radius-base 0 0;
   }
 

components/style/themes/default.less

@@ -394,6 +394,7 @@
 // --
 @modal-body-padding: 24px;
 @modal-header-bg: @component-background;
+@modal-header-border-color-split: @border-color-split;
 @modal-heading-color: @heading-color;
 @modal-footer-bg: transparent;
 @modal-footer-border-color-split: @border-color-split;