Digging deeper I can see what the issue is. The site.css contains the following styling for anchor tags
a {
color: #8a2445 !important;
text-decoration: none !important
}
The link colour overrides the button text colour, and I guess my link colour is the same as my button colour. I find the override situation a bit odd, considering most buttons are links. Why have a button text colour setting if it is never going to be used?
EDIT: I am really forgetful! The problem is caused by custom CSS that I myself added as custom CSS. Doh! Now if I can just remember why I added that custom CSS 😛
