Hi there,
try this CSS:
.offside--left.is-open, .offside-js--is-left .offside-sliding-element {
-webkit-transform: translate3d(357px,0,0);
-moz-transform: translate3d(357px,0,0);
-ms-transform: translate3d(357px,0,0);
-o-transform: translate3d(357px,0,0);
transform: translate3d(357px,0,0);
}
.offside--right.is-open, .offside-js--is-right .offside-sliding-element {
-webkit-transform: translate3d(-357px,0,0);
-moz-transform: translate3d(-357px,0,0);
-ms-transform: translate3d(-357px,0,0);
-o-transform: translate3d(-357px,0,0);
transform: translate3d(-357px,0,0);
}
.main-navigation.offside {
width: 357px;
}
.offside--left {
left: -357px;
}
.offside--right {
right: -357px;
}