My Mom’s house is a dark-brown stained one story contemporary with a garage on the front (yuck!) and a skylight in the living room. I am confused about helping her redecorate b/c the person who built it has everything from a fancy chandelier in the entry way to chair rails, floral wallpaper, and a wood-panelled den. How do I know what style is best for redecorating? Should a home that is contemporary on the outside also be so inside?

It is nice to decorate in the style of the home. There is a continuity of flow throughout. Many people however do not. They dress there homes in a style with which they are comfortable. It would be off to see a Victorian home decorated in an Asian design but whatever floats your boat I suppose.
not necessarily you do not have to go contemporary on the inside go whatever her taste is.