Change Close (exit) Icon on gallery pictures

Good day everyone,

I was wondering if we can change the close (exit) icon for gallery page once pictures are clicked? At the moment it's 'X' we want it changed to squares if possible. Like so:

Link: https://www.ptphotography.com/stmoritz-badrutts-palace-hotel-switzerland?itemId=9la391dhn5ba8t9britkx9bk894e4b

Thank you I advance,
K

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