Supporting multiple themes on the same screen would be very difficult.
You can easily assign a CSS class to a table but after some tests I found that the CSS class could only be used to style the table form. Assuming you assigned the value customTableClass as the table CSS class you would use the following CSS to add a red border around the child form.
.ui-tabs-panel .detailsForm.customTableClass {
border: red 1px solid;
}
I will be making an update to the next 1.2.1 release of dbFront to fix this issue so that you can also style the child table grid. I will update this question with further details once I have completed the fix.