If you need to make your subtotals look different than your data, it's not very apparent how to do so. I've found 2 ways.
You can conditional format the data cell and the results will affect the subtotals. The trick is to use INSCOPE. What you are going to do is check and see if the cell is in the scope of your group. First you will need the name of the group (right-click on the first field of the detail row and choose "Edit Group", you'll find the name there). In my case, the group name is "matrix1_Market_Name". Then you need to edit the cell you want to format the subtotal on, choose expression for font weight:
This will make the subtotals bold while the data will be normal. You can conditionally format anything like that. Here is font size:
You could do much more sophisticated stuff like change colors based on cell value with different criteria for the data and subtotal cells.
I've showed you the hard way, there is an alternate way which is much simpler. It not very apparent. If you right-click on the small green triangle in the subtotal label cell, then select properties the properties window is actually for the subtotal cell, not the label.