I want to add a shadow overlay to my website [closed]

I want to add a shadow overlay to my website CSS code over the whole page , I already downloaded the shadow file that I uploaded for you guys to check. It's in eps form and i don't know how to insert it into css. If anyone knows the code line please let me know. here is the file link: https://www.filemail.com/d/puxttluozcdaqah