If you installed the eZ Web Interface package (you most likely did), the file is located in the /var/storage/packages/ez_systems/ezwebin_design/files/default/file/ folder. This is where the default site style package is located.
You can also create a new Site Style package using the Package Manager in Setup to manage your add your own css overrides and images. Then you can use Site Settings to change the site to use your new site style package.
