You would use the design manager under marketing > files and Templates > Design Tools.
You can either edit the template itself that you are working on, or a better solution would be to create a module and add it to the template (or drag it onto the page with hubspot's drag and drop editor).
If it's something very small you can write html and css directly into a "Rich Text" module that you put on a page by clicking advanced > source code in the module editor. I would not recommend this method unless it's a very minor change though.
