Hi @HaoChan ,
As per my knowledge, I'm afraid it is not supported in SharePoint form layout configuration. Unlike the header and the footer, body configuration only allows defining one or more sections and adding one or more columns into each of those sections. We cannot add "style" code to body.
You can provide feedback here: https://feedbackportal.microsoft.com/feedback/forum/06735c62-321c-ec11-b6e7-0022481f8472. Microsoft will take your feedback into consideration if it is highly voted.
----------
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.