Can't get the CSS to apply to my story in Harlowe

Hey, I'm using Harlowe on the Twine 2 Mac app, and none of the styles I've put in the stylesheet seem to be applying to the actual story. I ended up moving over to Snowman, where it worked fine, but I don't really have time right now to get to grips with all of the script I'd need on there. So I want to use the macros on Harlowe, but it won't let me change any of the defaults.

Is this a known issue? I've tried looking for information about whether I'm meant to use different syntax for Harlowe in CSS but I couldn't find anything.

So none of this (or any of the rest of the CSS) is applying:

body {
font-family: Helvetica, Arial, sans-serif;
font-size: 16px;
margin-top: 100px;
background-color: grey;
font-weight: 550;
}

Edit: Just imported it into the web editor and I have the same problem. So confused! Tried putting this styling into passage {} as well with no luck.