Hi there!

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Please use the following code in your custom CSS to change the offcanvas menu icon and the overlay background color to make them visible:

.burger-icon>span {
    background-color: black;
}
.offcanvas-overlay {
    background: white;
}

Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Best Regards