For the preview MaxBulk Mailer uses WebKit, the Safari HTML rendering engine. Have you tried to reset the Safari Cache? Note that MaxBulk Mailer uses no cache at all.
I don't know then, MaxBulk Mailer has no cache and the Preview is made with Safari rendering engine, MaxBulk Mailer has almost no code for that, it is a direct call with no parameters. Try to restart your computer.
No, restarting my computer shows no change. It really looks like if the version of the css file ist still used which was current at the first preview of this document.
What about if you try to reproduce this issue? Can you see the stylesheet changes in your preview?
I use CSS all the time and I never had any issue, I sent a mail to all MaxBulk Mailer users a few days ago with CSS. CSS is nothing more than text that defines the document actually.