You would override the twig file and either include the overide.css file, or you would add your CSS directly to that.
You override a twig file by making a copy of the original in to your theme directory in the same location as the original, and then modifying it. The docs I linked do explain that!
You can get the original twig files from GitHub - but I’d strongly suggest only overriding the ones you specifically need otherwise maintaining your theme each release will become a huge task!
assuming 2.3 series. If you’re going to modify the login.twig, then please be sure you understand your obligations under AGPLv3 particularly with regard to the Source and About links. They are there to ensure compliance. If you remove them then you will need to make alternative arrangements to ensure you comply.
