Modify the width and color

Hi everybody,

I’m a new user in this forum from Catalonia (Spain). I used to write in a WordPress blog from the free WordPress service in WordPress.com

Now I want to go a step further and I have the WordPress version 3.0.5 in my own domain: danielarenas.enfoca2.com. I have installed the “freshy2” theme from http://www.jide.fr/english/downloads/freshy2 (open source). But I want to do some changes. I have had a harder search in the Net and I learnt that I need to make some changes in the CSS. I searched for specific information about how to change the width, and the colour of the background and the text but I have no succeed.

Anyone can help me in how and what I have to modify in my theme to have the background of the page in black colour and the text of the entries in white?
I also want to change the width of the useful area, I wanted to use the most part of the screen for the text and the menus (it will be a photographic blog). Could anyone help me?

It’s posible to do that using a plugin for wordpress?

Thank you so much and I’m so sorry for my English!