Hi envira,
Any template that MyNetwork uses can be found in the MyNetwork Templates template group. Go to your ACP -> Themes & Styles -> Templates (in the menu on the left) -> MyNetwork Templates. Even if they seem just like the default templates, they are what MyNetwork uses!
Stylesheets are added to each theme whenever you install something. For example, if you install the Profile System, it will add a profile.css stylesheet to each of your themes.
If you could tell me what kind of thing you're wanting to customize, I could let you know in a bit more detail.