but this does not explain how new CSS files can be imported into Hubspot. Grateful for assistance. The CSS files have already been created outside Hubspot.
You can import css into hubspot by creating a file (like in the article you shared), clearing any default css, and pasting your css into the file.
You can attach css to a template using the edit tab of the left side bar in the template editor. Make sure that no modules are selected.
You can attach css to a specifc page using the settings tab of the page editor.
You are able to attach css files globally to your portal in the website area of your portal settings, though I find this to be bad practice.