messy dropdown menu in checkout page

I would really appreciate any help for this, I tried some custom css to fix this dropdown menu but nothing really seems to be working.
The dropdown menu of checkout page of woocommerce is overlapping with other field in the billing details, It is transparent so you cant really see what are the options but when I gave background-color the other fields is hiding behind it. pls see in the link above.

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