Transparent Borders

snippet

#lightbox { 
        
            background-color: white;
            
            -moz-background-clip: padding;     /* Firefox 3.6 */
            -webkit-background-clip: padding;  /* Safari 4? Chrome 6? */
            background-clip: padding-box;      /* Firefox 4, Safari 5, Opera 10, IE 9 */
                         
            border: 20px solid rgba(0,0,0,0.3);
                        
            -webkit-border-radius: 40px;
            -moz-border-radius: 40px;
            border-radius: 40px; 
}

/*it's all in the background-clip property, from css-tricks.com*/