How to use your own inline css

function tifd_feedzy_custom_style( $content, $feedURL ) {
    $customStyle = '
            .feedzy-rss .rss_item {
                border-bottom: 1px solid #eee;
            }
            .feedzy-rss .rss_item .title {
                font-weight: bold;
            }
            .feedzy-rss .rss_item:after{
                content:"";
                display:block;
                clear: both;
            }
            .feedzy-rss .rss_item .rss_image {
                float: left;
                text-decoration: none;
                border: none;
            }
            .feedzy-rss .rss_item .rss_image span{
                display:inline-block;
                background-size: cover;
                background-position: 50%;
            }
            .feedzy-rss .rss_item .rss_image {
                margin: 0.3em 1em 0 0;
            }
            .feedzy-rss .rss_item .rss_content small {
                display: block;
                font-size: 0.9em;
                font-style:italic;
            }
        ';
    return $content;
}
add_filter( 'feedzy_global_output', 'tifd_feedzy_custom_style', 9, 2 );