The Portal code runs in an iframe. You can’t easily restyle it with css. If you’re self hosted, you can load a custom version of the portal that’s styled as you want it. (Not an option on Ghost Pro and other managed hosts.)
However, you /can/ turn it off. Many themes offer custom sign-in/sign-up pages, that run on the theme level and /can/ be customized. For example, Liebling has a custom newsletter sign-up: https://liebling.eduardogomez.io/ (free theme!). Many of @fueko 's themes (not free) include custom membership pages, such as this one: https://eihei-dark.fueko.net/ (click the join button)
Not disagreeing that there should be an easy way to tweak the display of Portal, but there are some options, at least. 