bualai

I’ve been able to solve the problems. I will write the solution here so that it might help somebody in the future.

The “Login” link had such a big hover area because I used the .login class, which apparently is already present in FlatUi registration theme inside the plugin. The plugin class overrode my styles with a huge padding area.
To solve it, I simply changed my class from .login to .link-login

The “Sign up” button didn’t work because for some reason it was under div.container.
To solve it, I added

position: relative;

to the style of its <div>.