Snippets: Change Sidebar Mobile Menu Dropdown Arrows to Plus Icon

/* Show plus icon by default */
body .sidr-class-dropdown-toggle:before {
    content: "\002B";
    font-family: "Arial";
    font-size: 16px;
}

/* Show minus icon when active */
body .sidr-class-dropdown-menu li.active > a > .sidr-class-dropdown-toggle:before {
     content: "\2212";
}

Result:

All CSS snippets should be added via a child theme's style.css file or using the built-in Custom CSS panel.