Hamburger background fade


I have a background that applies to the hamburger menu when the X appears. How can I get that background to fade in/out?

Thank you!

Here is my site Read-Only: LINK
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I’m actually looking to do something like Starbucks does on their hamburger menu where the hover on both open and close has a background color that gowns in and out:

No one? I have tried for days any tips would be greatly appreciated thank you!