You can add CSS to your theme, or if your theme doesn’t allow customizations because it’s a default theme or hosted remotely, you can create a theme component and add that to your theme (Customizing themes without an Edit code button)
I am not sure if that also works for page publishing. Canapin said it doesn’t
but maybe he meant it more generally and less focused on CSS. I found Published Page CSS Not Showing on Frontend - #7 by david which makes me think that it should work