Where are the Multisite files/folders? (local install)

I am using WordPress 3.0.1 Multisite on my laptop (local installation).

I created several subdirectories (different websites) and trying out different themes. I want to change the color of one of the themes. Where are the css files for the theme? Normally it is in the htdoc/wordpress/wp-content/themes location, but for Multisite I am not sure where they are.

If the file is in the htdoc/wordpress/wp-content/themes folder than if I modify the theme, it will apply to all sites that use that theme.

The css file address looks like:
https://localhost/wordpress/website-1/wp-content/themes/theme-abc/style.css