Found the problem. The skin's own CSS has a color for
button, but it's not limited to the skin's own buttons. It's cascading down to the widget's button. Might be very tricky to correct the skin so that it's not generating CSS that spills over like this.
Adding this to
Projector > Advanced > Custom code > CSS will, I believe, fix the problem:
#jalbum-submit {color: black;}