Change Close (exit) Icon on gallery pictures

Add to Home > Design > Custom CSS

.gallery-lightbox-close-btn-icon {
    visibility: hidden;
}
.gallery-lightbox-close-btn-icon:after {
    content: "\e014";
    visibility: visible;
    font-family: 'squarespace-ui-font';
    font-size: 20px;
}

@tuanphan thank you so much for this! Thank goodness it is possible to change the icon. 

I have one last favour 🙏

At the moment we have this:

I see that when you change the content number it shows a different icon, but no matter how hard I try, I can't find the letter and number combination to achieve this: All 4 squares filled background.

Please help updating the code once again!

Thank you,
K