How do I alias a class name in C#, without having to add a line of code to every file that uses the class?

You can’t. The next best thing you can do is have using declarations in the files that use the class.

For example, you could rewrite the dependent code using an import alias (as a quasi-typedef substitute):

using ColorScheme = The.Fully.Qualified.Namespace.Outlook2007ColorScheme;

Unfortunately this needs to go into every scope/file that uses the name.

I therefore don't know if this is practical in your case.

You can make an alias for your class by adding this line of code:

using Outlook2007ColorScheme = YourNameSpace.ColorScheme;

You want a (Factory|Singleton), depending on your requirements. The premise is to make it so that the client code doesn't have to know which color scheme it is getting. If the color scheme should be application wide, a singleton should be fine. If you may use a different scheme in different circumstances, a Factory pattern is probably the way to go. Either way, when the color scheme needs to change, the code only has to be changed in one place.

public interface ColorScheme {
    Color TitleBar { get; }
    Color Background{ get; }
    ...
}

public static class ColorSchemeFactory {

    private static ColorScheme scheme = new Outlook2007ColorScheme();

    public static ColorScheme GetColorScheme() { //Add applicable arguments
        return scheme;
    }
}

public class Outlook2003ColorScheme: ColorScheme {
   public Color TitleBar {
       get { return Color.LightBlue; }
   }

    public Color Background {
        get { return Color.Gray; }
    }
}

public class Outlook2007ColorScheme: ColorScheme {
   public Color TitleBar {
       get { return Color.Blue; }
   }

    public Color Background {
        get { return Color.White; }
    }
}