Hi wftl and welcome to the RH community. I find it easier to apply a style sheet to the topic's properties. It means no messing around inside the HTML and you can apply the .CSS across multiple topics at a time by selecting them.
Sadly, we have hundreds of topics which would make applying an external style sheet via topics a daunting proposal.
Over to you, and thanks.
Nay, not so!
Just select every appropriate topic in the Topics tab, right click and select Properties, and assign the external style sheet in the Appearance tab.
Make sure you do this from the Topic List tab at the bottom of the screen and NOT from the Project Manager tab at the left. You will save yourself hours of work if you select all of your files in the Topic List at one time and just apply your one CSS file once!