Suppose you want to change background of bundle product button then write down below style in your less file 
.bundle-actions {
    .action.primary.customize {
        background-color: @button-background;
    }
}

Suppose you want to change background of bundle product button then write down below style in your less file

.bundle-actions {
    .action.primary.customize {
        background-color: @button-background;
    }
}