Official Facebook CSS styling guide?

It appears to me Facebook is using different CSS styles for:

- Comments to typical FB posts
- Comments to posts in FB groups
- Comments using the FB comments plugin

Some of the differences I see:

- Backgrounds are white, light grey or dark grey
- Profile pictures are round or square

Is there an official page where the current CSS styles Facebook uses are seen?

Or is there something else going on here (browser issue maybe)?

Thank you in advance.

I don’t think so. Facebook has absolutely no reason to do this. Not even google does it and google has Material Design.