You may change the background of your menu using either a solid background color or an image. Not all menu templates has an option to change the menu background color, the option to change the background color of your menu is only available using box menu templates.
Here are 2 ways to change the background of your menu in Portfoliobox:
Set an image as a background on your menu
You can do this by going to Design > Colors & Fonts > Menu > Background Image > upload the image that you wish to set as the background of your menu.
Set a colour as a background on your menu
To do this, please go to Design > Colors & Fonts > Menu > Box Color > select a color inside the color picker.
These are some examples of box menu templates that will allow you to change its menu background:
NB! Please be reminded that the menu background option will not be available using other templates aside from box template.