Does anybody know how to change/modify that login module ? (background-color ; input[type="text"] )

In k2.css they removed classes for that ? : "Modules - mod_k2_user (mod_k2_login is removed since v2.6.x)"

I tried override #login-form from K2 in custom.css - but also without success.

Which login module are you using?
the mod_login or K2's user module?