Hi,
one of your joomla template's CSS file forces that button to have a black text color.
Try to add this CSS rule to flexibleVM2Global.css
div#WWMainPage div.cart-view a.continue_link {
color:#ffffff;
}
This should fix the issue. For the other buttons (register&checkout and Guest checkout buttons)
Add these as well:
div#WWMainPage div.cart-view div.FlexibleLoginBoxRight a.FlexibleResgiterButton {
color:#ffffff;
}
div#WWMainPage div.cart-view div.FlexibleLoginBoxRight a.FlexibleGuestButton {
color:#ffffff;
}