where to adjust the site style for exit button on shop lightbox

Site URL: https://www.lonniwong.com/shop

Hi,

I am having troubles with a simple exit button for quite a long time. Its white on white and i have tried and digged in colors but i dont get it. Any help apreciated! 

Its only in the shop when you click a picture i choose lightbox. To close the images the "X" is really hard to see as it is white on white. 

How can i make the X black please?

for example this one https://www.lonniwong.com/shop/p/shop/?itemId=dwn7466jupkemyoln0l941czahaow7

thank you so much @tuanphan!!