css: add background color variable for modal mask. (#5890)

components/modal/style/modal.less

@@ -116,7 +116,7 @@
     left: 0;
     bottom: 0;
     background-color: #373737;
-    background-color: rgba(55, 55, 55, 0.6); // lesshint duplicateProperty: false
+    background-color: @modal-mask-bg; // lesshint duplicateProperty: false
     height: 100%;
     z-index: @zindex-modal-mask;
     filter: ~"alpha(opacity=50)";

components/style/themes/default.less

@@ -247,6 +247,10 @@
 //** Popover outer arrow color
 @popover-arrow-outer-color: fadeout(@border-color-base, 30%);
 
+// Modal
+// --
+@modal-mask-bg: rgba(55, 55, 55, 0.6);
+
 // Progress
 // --
 @process-default-color: @primary-color;