If you use ExtJS, you will always inherit the proper styles - simply add an xtype: 'button'.
If you'd rather use plain HTML, here's what a default button (in a toolbar) with ExtJS looks like:
<table id="ext-comp-1059" cellspacing="0" class="x-btn x-btn-noicon" style="width: auto;">
<tbody class="x-btn-small x-btn-icon-small-left">
<tr>
<td class="x-btn-tl"><i> </i></td>
<td class="x-btn-tc"></td>
<td class="x-btn-tr"><i> </i></td>
</tr>
<tr>
<td class="x-btn-ml"><i> </i></td>
<td class="x-btn-mc">
<em class="" unselectable="on">
<button type="button" id="ext-gen84" class=" x-btn-text" style="">Text In Button</button>
</em>
</td>
<td class="x-btn-mr"><i> </i></td>
</tr>
<tr>
<td class="x-btn-bl"><i> </i></td>
<td class="x-btn-bc"></td>
<td class="x-btn-br"><i> </i></td>
</tr>
</tbody>
</table>
.. pretty huh?
You could also borrow/adapt the styles from
http://chsmedien.com/blog/2013/02/customize-your-modx-dashboard which mimics the buttons but has a much, much cleaner syntax.
Revo 2.3 will have a different design though - the benefit of using ExtJS is that it will carry along nicely when 2.3 lands.