Looks like you added the css given in the tutorial into http://www.boucheabouches.fr/wp-content/plugins/contact-form-7/includes/css/styles.css - a file within the CF7 plugins directory.
The tutorial doesn't spell it out, but you should not be adding the css from the tutorial directly into the CF7 plugins css files. If you do then any changes will be lost whenever you update the plugin.
Rather the most common and preferred method is to create a Child Theme and make any necessary additions to the CSS in the child themes style.css file. That way you also don't loose your changes when you update your WordPress theme.