Button height issue in Revolution Slider with Chrome and Safari

Hi there!

If I uses this code for a button in the revolution slider, the button does not have the right height in Safari or Chrome (Firefox works fine):
<a href="/wordpress/referenzen" class="button dark_btn sm_button">Referenzen</a>

It’s the second slide on http://reprobau.de/wordpress/

What can I do to fix this?