Hover/animation effects on navigation links

Site URL: https://wombat-dinosaur-kkfh.squarespace.com/config/

Hello!

I desperately need help finding a CSS code that will work for a hover effect on my navigation links in the menu bar. I want the effect to be something like an underline when the mouse is over a specific link OR an effect where all the links blur when the mouse is hovering over a specific link. 

Any help would be SO appreciated I have literally tried everything

Thank you!!!! 

Your site is private, please set to password and post the password.