color change of lightbox exit (x) sign possible?

Site URL: https://leopard-owl-xw5g.squarespace.com/

Hello,
im trying now since some days to figure out, how to change the lightbox exit sign visible. Tried using different color themes, but i need some help pls.

How can i simply change the X of the lightbox color to black ? Its barely visible right now.

https://leopard-owl-xw5g.squarespace.com/
pw: testme

Thank you for your help!

Where to see X icon?

https://leopard-owl-xw5g.squarespace.com/home