basic theming question on user module I would like to ask a very simple (for you probably!) question on theming/layout:

By default the user module displayes the "username" + "password" vertically.
I want to display these 2 fields horizontally. 

To do so:

-It is the css that I have to modify, right?
-Do i have to modify the user.css or the css of my theme?
-Any ideas on how do I modify it, or what is the piece of code I have to alter?

Thank you.
You can extend Omega Theme and override the css in your global.css file. So you will have  a new theme and core files are intact.

All you need to install is  Omega Theme. And the By going to Appearence you can create a new theme and override the css files.