Installing a new theme is usually just as simple as unzipping it to the theme folder, then going into bigbox options and switching to it. Nothing will be overwritten at all and the new theme will use all the images and video's and such that the default theme does. Also most of the newer themes come with a settings .xml that should automatically change any specific settings that the theme in question needs, and if you don't like the theme just switch back and delete the folder from your theme folder.
As for best that is very subjective, i personally like critical zone (which you already have as it's built into launchbox these day's) and i regularly use @Grila fundamental theme, it's really clean with big box art.