Set alternating post classes, without the use of columns="x" in the foreach loop


So I’d like to use a feature similar to the columns attribute of the foreach block. But if I also want to have a masthead featured post, then the first post in my list thinks it’s the end of a row, without there being a start of the row.
So I wondered if there was a way I could have an alternating class thing, or if there was a way to set up a CSS grid column system with nth-child().

Thanks in advance,