Changing Theme Without Losing Site Info

Hi, I am working on a client’s website and I’d like to know if I can change the theme/template without losing anything on the client’s website? And how?

Current website has a black background with grey and white text. I need to change it to a white background with black text.

Thanks for any help in advance!
Website I am Editing: http://www.prestigekitchenbath.com/

closed as duplicate http://wordpress.org/support/topic/activating-new-wordpress-theme?replies=2#post-3819263