CSS – How to make my white header semi white transparent

Hi all, my theme developer does not have an option to create a semi-transparent header. Only solid or full transparent. Is there a css fuction that I can use to make my banner semi transparent (either semi white or semi dark) – thanks so much!

Here’s the site url.
https://45e.a19.myftpupload.com/home-builder-demo-normal-navigation/