Open the themes gtk3 css file both the normal and the dark and find:
.budgie-panel {
transition: background-color 75ms cubic-bezier(0, 0, 0.2, 1);
background-color: #212121;
color: rgba(255, 255, 255, 0.7);
font-weight: 500;
}
.budgie-panel.transparent {
background-color: rgba(33, 33, 33, 0.65);
}
And adjust them to your desire.
EDIT: Wrong lines, I need some sleep 😛 . You need to edit:
.budgie-popover {
border: none;
border-radius: 10px;
box-shadow: 0 3px 3px -2px rgba(0, 0, 0, 0.2), 0 3px 3px 0 rgba(0, 0, 0, 0.14), 0 1px 5px 0 rgba(0, 0, 0, 0.12);
background-color: rgba(33, 33, 33, 0.65);
border-color: #2C2C2C;
}
the background-color you have to look at.