You need to first remove the infowindow background & shadow, then apply your own css.
JS part
/*
* The google.maps.event.addListener() event waits for
* the creation of the infowindow HTML structure 'domready'
* and before the opening of the infowindow defined styles
* are applied.
*/
google.maps.event.addListener(infowindow, 'domready', function() {
// Reference to the DIV which receives the contents of the infowindow using jQuery
var iwOuter = $('.gm-style-iw');
/* The DIV we want to change is above the .gm-style-iw DIV.
* So, we use jQuery and create a iwBackground variable,
* and took advantage of the existing reference to .gm-style-iw for the previous DIV with .prev().
*/
var iwBackground = iwOuter.prev();
// Remove the background shadow DIV
iwBackground.children(':nth-child(2)').css({'display' : 'none'});
// Remove the white background DIV
iwBackground.children(':nth-child(4)').css({'display' : 'none'});
});
CSS part (example)
.gm-style-iw {
width: 350px !important;
top: 0 !important;
left: 0 !important;
background-color: #fff;
box-shadow: 0 1px 6px rgba(178, 178, 178, 0.6);
border: 1px solid rgba(72, 181, 233, 0.6);
border-radius: 2px 2px 0 0;
}