Using DTRegister 2.5.1b, on the page that allows a user to select his or her payment method, the "Next Step" button is without a style coding. It shows up as a regular button instead of using the template style. It just needs a class attribute to be added.
For a fix, open dtregister.php and around line 7247 you will see...
<input type="button" value="<?php echo JText::_( 'DT_NEXT_BUTTON' ); ?>" name="submitbtn" onClick="paymentform();"></td></tr>
Just change it to...
<input type="button" class="button" value="<?php echo JText::_( 'DT_NEXT_BUTTON' ); ?>" name="submitbtn" onClick="paymentform();"></td></tr>
and it's fixed.
While you're at it, see line 6901 and add the same
attribute to that tag, too.