Skin File and CSS

I am very new to ASP.NET so I am a little lost in regard to how the .skin file that is included in a few of the app themes folders (i.e. darkOrange) works with the CSS. I want to be able to manipulate the CSS file to change the look and feel of my store but am clueless as to how the .skin file affects the CSS file or the store for that matter. Can anyone enlighten me about how these files work together to make the store and how to customize the store with these files. Is the .skin file necessary? Thanks in advance for any assistance.

easiest way to start is to make a copy of one of the existing theme folders ( but you can also start from scratch with a new theme, recommend your first customisation is by editing en existing theme though)

rename the folder - you choose the theme name

in admin configuration-->global settings, select your new theme name and save - your storefront will look exactly the same as the theme you copied

browse - use firefox browser with firbug installed ( start firebug with crtl+shift+c ) - this will let you examine all the elements on the page and will pinpoint exactly where in the css you need to look to edit

you can also edit the images in the theme images folder

eg to change the logo, go to

app themes --> theme name --> images

look for logo.gif