Help please with 7.1 update

Hi we have been using squarespace for a long time but recently decided we should update to 7.1. All was well until i realised today that our hamburger menu is invisible. It is there but not seen. I spent an age going through colours but am stuck. Website www.dorsetwatersofteners.com

Because burger icon is transparent color

You can add this code to Website Tools > Custom CSS

div.burger-inner>div {
    background-color: black !important;
}

Thank you very much i assumed that it was transparent but could not figure out why. Tried the code and it works so a big thank you