Web: Add purecss/buttons.css.

Change-Id: I0c4b049de17629510619ff9fcca3f0bd263a93b5
diff --git a/src/main/resources/META-INF/resources/package.json b/src/main/resources/META-INF/resources/package.json
index c61cf2f..97d56bc 100644
--- a/src/main/resources/META-INF/resources/package.json
+++ b/src/main/resources/META-INF/resources/package.json
@@ -18,7 +18,7 @@
       "sanitize.css/{sanitize,forms,formsize,typography,page}.css",
       "prefixfree",
       "prismjs",
-      "purecss/build/{base,forms,menus,tables}.css",
+      "purecss/build/{base,buttons,forms,menus,tables}.css",
       "redux"
     ]
   }