To be more specific:
Add &levelClass=`button` and &rowTpl=`rowTplChunk` to your Wayfinder call.
The rowTplChunk should be something like this:
<li class="[[+wf.classnames]] has-sub"><a href="[[+wf.link]]" title="[[+wf.title]]">[[+wf.linktext]]</a>[[+wf.wrapper]]</li>
So the menu items of first level are output like this:
<li class="button1 has-sub"><a href="/" title=""><span>First level</span></a>....</li>
and the items of second level like this:
<li class="button2 has-sub"><a href="/" title=""><span>Second level</span></a>....</li>
and so on.
Now you need your dirty CSS:
ul.navigation li.button1 > a {
/* put here the rules you used in your class button */
}
You don't need any rules for the li.button2 and so on; they are just ignored.