I had the same question. What Lisa said was true, the border and border color is available for columns but not (as far as I’ve seen) for anything else. I wanted to put a border on a paragraph but it must be getting blocked in core code. I tried to overwrite it in theme.json but that did not work either. Still working on my understanding of wordpress theme development.
UPDATE: the only solution I have found is to select the code editor and add the border code right into the <p> tag. e.g. <p style=”border:orange;?border-width:5px;?border-style:solid;”>
