How can I add some css in the login module (K2 user) of k2? Which file should I edit?

I mean in the "Forgot your password?"

Hello bobis,

It's k2.css.

From k2.css I change only the css of the Login, username and password. My problem is when I add css in the left menu of my website add also css in the "forgot you password", "forgot your username?" and the "Don't have an account yet?". I want to separate the left menu with the "forgot you password", "forgot your username?" and the "Don't have an account yet?". How can I do that?

You can use module suffixes or use the container class from your template.