Change burger menu icon for a custom icon desktop

Site URL: https://copper-bullfrog-28wb.squarespace.com

Hello! Does anybody know how can I change my default burger menu icon for a custom one?

I have done all I know and I can only manage to change the phone version, but I want to change the DESKTOP one.

I have read about the icon being /e30/ or something like that, but when I upload my custom .png icon, it doesn't change at all.

PASSWORD: EspenWebsite2022

Thank you so much in advance!

Add to Design > Custom CSS

a.header-nav-folder-title:before {
    color: transparent;
}
a.header-nav-folder-title {
    background-image: url(https://cdn.pixabay.com/photo/2022/03/01/00/12/tree-7040298__340.jpg);
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center center;
}