Hi Patrick,
The CSS of your jQuery UI Theme is:
https://code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css
If you look for the lines:
.ui-state-active .ui-icon,
.ui-button:active .ui-icon {
background-image: url("images/ui-icons_ffffff_256x240.png");
}
you can see that the icon is an image (actually it’s part of a larger image).
To replace it, try adding to Customize -> Additional CSS the following lines:
.ui-state-active .ui-icon {
background-image: url("https://code.jquery.com/ui/1.12.1/themes/base/images/ui-icons_444444_256x240.png") !important;
}