How can i add a background for drop down menu in navigation bar?

Hello,

i hope someone can help me,

please look at the navigation bar of this site
https://www.almuroojprivateschool.com/
the background of the drop down menu is transparent
and it makes it hard to read.

can someone tell me how can i add a background color to the
drop down menu?

thank you in advance i really appreciate it.:)