For anyone else trying to workaround not being able to add styles directly, you also CANNOT add them using Google Tag Manager! I just lost many hours troubleshooting why my HTML tags embedded with GTM weren’t firing.
There is a note about this on one help page, but unless you go looking for it you wouldn’t know. There warning in the backend where you enter your GTM container ID (but there is a link to a help document, but no explicit warning).
The most ideal solution would still be a custom CSS back in the backend as GTM is blocked by some adblockers.