Your primary menu container is set to left: 5000px; witch stretches the width of the page. You can find this style directive in …themes/sunstone-bookkeeping/css/mobile.css on line 66.
@media screen and (max-width: 600px)
.nav-menu {
position: absolute;
left: 5000px;
}
One way to fix this is to add the following styles to the parent element:
.menu-menu-1-container {
position: relative;
overflow-x: hidden;
}
This solution shouldn’t mess with other styles.
The proper way of applying this kind of changes is described here: child themes. In the style.css file of your child theme you can safely add the solution that I proposed. Cheers!