Created a new `reboot-bootstrap-modal.scss` and changed the color specification method of $ grw-moda-head.

src/client/styles/scss/_layout.scss

@@ -16,7 +16,7 @@ body {
 
 .grw-modal-head {
   font-size: 1em;
-  border-bottom: 1px solid $border-color;
+  border-bottom: 1px solid $modal-header-border-color;
 }
 
 // padding settings for GrowiNavbarBottom

src/client/styles/scss/theme/_apply-colors-dark.scss

@@ -22,9 +22,11 @@ $table-dark-bg: $bgcolor-table;
 $table-dark-border-color: $border-color-table;
 $table-dark-hover-color: $color-table-hover;
 $table-dark-hover-bg: $bgcolor-table-hover;
+$modal-header-border-color: $border-color;
 
 @import 'reboot-bootstrap-border-colors';
 @import 'reboot-bootstrap-tables';
+@import 'reboot-bootstrap-modal';
 
 // List Group
 @include override-list-group-item($color-list, $bgcolor-list, $color-list-hover, $bgcolor-list-hover, $color-list-active, $bgcolor-list-active);
@@ -356,13 +358,6 @@ ul.pagination {
   border-color: $border-color;
 }
 
-/*
- * modal
- */
-.grw-modal-head {
-  border-color: $border-color;
-}
-
 .revision-toc {
   .top-of-table-contents {
     border-color: $border-color;

src/client/styles/scss/theme/_reboot-bootstrap-modal.scss

@@ -0,0 +1,3 @@
+.grw-modal-head {
+  border-bottom-color: $modal-header-border-color;
+}