
Hi @Muki ,
As far as I know, this is not supported by Microsoft currently. We can only add background-color or any css style to header or footer but not body.
Here is the document for more details
https://learn.microsoft.com/en-us/sharepoint/dev/declarative-customization/list-form-configuration#configure-custom-body-with-one-or-more-sections
I have found a similar issue with yours for reference
https://learn.microsoft.com/en-us/answers/questions/468039/list-form-json-format-body-section-width.html
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