open /includes/functions.php file.
search for
$lightbox_button
you'll find all kind of buttons code few lines down
replace for each button this code:
<img src=.......>
with your text.
there is a code that make buttons work like links, the code starts with:
<a href=.....>
and ends with:
</a>
so, in this case you will need replace <img src=....> that located between 'link' code
hope that helps.
