NOTE:
This will only work for the non-conversation view. It doesn't make sense to have conversation view on one line and I'm not sure how you would do it. So, this is only for the non-conversation view.

SETUP:
You may want to add in your own OWA server to apply the style to, but this isn't necessary. You should only need to do this if it affects other sites as well. I think the selectors are specific enough that it shouldn't affect other sites, but you never know. Select "URL's starting with" and for the URL put in the URL to your instance of OWA (ex. https://outlook.myserver.com/owa).