Well, here's some examples cool themes/layouts for your Google+ account:
Harry Potter Google+ layouts/theme
Lady Gaga Google+ layouts/theme
Blue sky Sunset Google+ theme/layout
Once you have installed the plugin, then to change your Google Plus background to any image you want just simply click the 'Edit Background' button in the lower right corner to change your background.
To install Layouts for Google Plus™ chrome plugin click here, then to browse everyone's background images visit this link: http://www.google-plus-themes.com/