Well, in my case i did not use a custom theme or icons. After installation i was missing icons both from menu items and launch panel below. I managed to get icons in the launch panel by editing items and linking to icons. However, i do not know where to edit the menu items?
I'm guessing you used the same homedir as you did in a previous installation of another distribution, one where you had selected to use another icon theme. It probably should have been enough to just select the mate-brave icon theme from the xfce appearance settings.