You are welcome 
Window Button icons I think the others you're wanting to change are called.
Getting the launcher icons how I want is usually enough for me & I've never worried about trying to change the Window Button icons.
A quick search of the Xfce forum shows possibly it can be done, but might be somewhat complicated...and I guess there's always the possibility messing with it can break things.
Someone might know an easy way to make these changes.
Good luck 8)
EDIT: search I linked to had expired but I searched for the words 'change window button icons' and browsed the results.
Window Button icons I think the others you're wanting to change are called.
Getting the launcher icons how I want is usually enough for me & I've never worried about trying to change the Window Button icons.
A quick search of the Xfce forum shows possibly it can be done, but might be somewhat complicated...and I guess there's always the possibility messing with it can break things.
Someone might know an easy way to make these changes.
Good luck 8)
EDIT: search I linked to had expired but I searched for the words 'change window button icons' and browsed the results.