Thanks for reaching out to Jotform Support. I tested the feature and I can see the Custom CSS section when clicking on the wand icon. Here's a screencast of my result:
Are you using a card layout? Unfortunately, it is not possible to inject CSS code with this layout. I suggest changing the form layout to the Classic Form Layout to be able to inject some custom CSS codes. Let me show you how to change the form layout:
In Form Builder, click the Roller Paint icon on the right side.
Select Layout on the Form Designer panel.
Click Classic Form.
Give it a try and let us know if you need any help.