Big Hamburger 2

It’s because of this CSS:

.js .main-navigation ul,
.main-navigation .menu-item-has-children > a > .icon,
.main-navigation .page_item_has_children > a > .icon,
.main-navigation ul a > .icon {
	display: block;
}

If you change it back to display: none, the menu will hide on smaller screens and appear properly when the menu button is clicked.

As a side note, if you edit the theme’s files directly, you’ll lose those changes if the theme is ever updated in the future. Instead, it’s best to use the built-in CSS editor at Dashboard > Appearance > Customize > Custom CSS, a custom CSS plugin, or a child theme.