Button: fix round button sizes

packages/theme-chalk/src/mixins/_button.scss

@@ -75,4 +75,7 @@
   padding: $padding-vertical $padding-horizontal;
   font-size: $font-size;
   border-radius: $border-radius;
+  &.is-round {
+    padding: $padding-vertical $padding-horizontal;
+  }
 }