Hi Alastair,
CSS classes are applied to components in different states. For the case of the button, .jqx-fill-state-hover when the mouse is over the button, .jqx-fill-state-pressed, when the button is pressed and .jqx-fill-state-focus when the button is on focus. I think that you will need to customize some of these classes and especially the .jqx-fill-state-pressed, which is applied when you click and hold the button.
Ex:
.jqx-button.jqx-fill-state-pressed {
// your css here.
}
Best regards,
Peter Stoev
jQWidgets Team
https://www.jqwidgets.com/