There is no way to do it via wordpress. Some people have had luck using the theme css but I myself had to edit the plugin css file (found in your ftp client in the plugins folder/ wp-customer-reviews/ wp-customer-reviews.css). Edit and save your css there.
If you update wp-customer-reviews plug-in your changes will be lost so save your css snippets to desktop incase you lose them.
update 3.0 is coming soon for the plug-in which will allow styles. Because wp-customer reviews runs scripts, with !important tag, the default css is always loaded last overwriting any changes you make.
