[improvement] move overlay component (#2192)

packages/style/base.less

@@ -14,3 +14,4 @@
 @import '../button/index';
 @import '../cell/index';
 @import '../cell-group/index';
+@import '../overlay/index';

packages/style/var.less

@@ -66,6 +66,9 @@
 @number-keyboard-key-height: 54px;
 @number-keyboard-key-background: #ebedf0;
 
+// Overlay
+@overlay-background-color: rgba(0, 0, 0, 0.7);
+
 // Radio
 @radio-size: 20px;