Well the CSS file only contains classes...most X-Cart sites are table based, and many developers put a lot of stying into the templates themselves. Again, it will depend on how your current site is coded.
The database doesn't contain any information on the design (other than some 'number of x per row' settings), so changing the templates and css file won't alter the database. I suggest making a backup of everything first, as I guarantee you will mess at least one template up before you are done
