How to change the background color of popupMenu selection item

[xcfdszzr]

[mgarin]

You can only change it via adjusting default menu item style or creating a custom one and applying to the specific item instance. If you aren't familiar with how WebLaF styling works - I recommend reading this wiki article:
https://github.com/mgarin/weblaf/wiki/Styling-introduction

The default style of menu item can be found here:
https://github.com/mgarin/weblaf/blob/master/modules/ui/src/com/alee/skin/light/resources/menuitem.xml