You have no logo and therefore when the menu collapses there is no header to show (which has a background on which the hamburger would normally reside over). So, just change the color of the hambuurger with custom SCSS.
.g-offcanvas-toggle {
color: red;
}
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Don't forget to recompile CSS from base outline after making any changes to custom SCSS file.
Regards, Mark.
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