Overlay Menu on mobile customizable? How to

I am a little bit new to making a website on square space. I have been trying to find this on google / youtube / forums for hours and I just cannot figure it out.

I don't like the way my overlay menu looks and the settings on squarespace aren't helping much.

My website is lustrerentals.com , I don't like the way my overlay menu takes the whole screen.

In the next picture it is another square space website tremontrentals.com , i like how their overlay menu on mobile only takes up half of the page.

Any help on how I can make my mobile overlay menu look like the one on tremontrentals.com would be appreciated!

Add this to Design > Custom CSS

@media screen and (max-width:767px) {
.header-menu {
    width: 70% !important;
}
div.header-menu-nav-folder:not(.header-menu-nav-folder--active) {
    opacity: 0;
}
}