Hello
Presuming I understand you correctly to change the background of the slideshow area of your template to white try adding the following to the Custom CSS field of your templates parameters...
#container_slideshow {
background: #FFFFFF;
}
The background to the logo image uses an image (
http://demo.joomla51.com/0095/templates/j51_clarity/images/bg_bar.png) which you will need to edit directly within your templates core files. You will find it located at ../templates/j51_clarity/images/bg_bar.png from the root of your Joomla installation.
Ciarán
