Whenever you install WordPress for someone and include themes for them to choose from, go through the following selection process:
First look at the list of compatible themes here: http://codex.wordpress.org/Themes/Theme_Compatibility/2.6
Then download the ones you like and upload them to a WordPress site either remotely or locally (with WAMP)
Make sure no plugins are enabled. Then go through selecting theme by theme and validating the (X)HTML and CSS with the links provided above. (Use the FireFox Browser and install the webdeveloper toolbar which has validation buttons)
If or when the validation fails with more than an obvious easily correctable mistake, don’t even bother, just delete the Theme and move on to the next.
This is not only a great way to avoid problems with your WP site but you’ll get a pretty good overview which Theme authors are good out there and really know what they’re doing. Eventually you keep an eye open for new designs by those authors because you can use their themes without having to do much customization if any.
Really do make sure you have all plugins deactivated during this process because if there’s an error in the code the plugin inserts into your page none of the themes will (seem to) validate.
When you select a plugin, also check if it doesn’t screw-up the validation. If it’s an important plugin for you, try to correct the error and/or contact the maker of the plugin to let him know.
You can also make your own theme, you know. Try it, if your attempt fails you’ll at least gain more respect for the hard work theme developers put into making their themes available for free.
Courtesy : http://wordpress.org