Hi Vera -
I took a look at the "Add to Cart" button in the demo X-CART store - and it is set up to be bold text. So, maybe you have a mod to make it plain text along the way somewhere?
If you have not changed the css class from the default - it is using Button - which in the default is defined:
font.Button {
skin1.css (line 712)
color:#490101;
font-family:Verdana,Arial,Helvetica,Sans-serif;
font-size:11px;
font-weight:bold;
text-decoration:none;
}
You can check your skin1.css to see the class that is defining your button. If you did get rid of the bold for a reason (probably many buttons share this class) - there are ways to edit button.tpl for a specific button - or, it may be easiest to do as Balinor suggested and make your own button.tpl. You can post or pm a link if you want more information about why your style is plain text there.