My CSS had:

.my-app-active .progress-bar.my-app-custom-progress .bar {
 -fx-background-color: green;
}
.my-app-custom-progress {
 -fx-border-width: 0;
}

When the custom classes ("my-app-coustom-progress", "my-app-active") were set on my ProgressBar control and the parent container and I was using the default "Caspian" styles, this showed a green bar for progress. With Modena the result is that the progress bar doesn't show any progress at all.