Try This CSS:
@media (max-width: 768px) {
.main-navigation.offside {
width: 200px;
right: -200px;
}
.offside--right.is-open, .offside-js--is-right .offside-sliding-element {
-webkit-transform: translate3d(-200px,0,0);
-moz-transform: translate3d(-200px,0,0);
-ms-transform: translate3d(-200px,0,0);
-o-transform: translate3d(-200px,0,0);
transform: translate3d(-200px,0,0);
}
.offside-js--is-right.slide-opened .slideout-overlay button.slideout-exit {
right: 200px;
}
}