Add to cart button is missing on product page

Hello guys,

Today I’ve found a bug on my website; on the product page, the add to cart button appears to exist (I mean the function to add the product in cart is still exist, but the button is invisible, like in this photo where you can observe it: https://imgur.com/tw9toYj)

I’ve tried all of these:

• change the button color from Appearance -> WooCommerce -> Single product
• turn off all the plugins to see a potential plugin conflict (I don’t have one at the moment)
• try to install a plugin who can change the color of the button to make it visible (it doesn’t work, the button is still the same as in the photo)

I am waiting for any opinion that could solve the problem! ASAP

The page I need help with: [log in to see the link]