When something "sets the tone" it simply means that that "something" influences the ambiance/atmosphere. If you arrive at a dinner party and the hostess is wearing something very elegant and formal...that "sets the tone."
If you choose to have a wedding ceremony at the beach, that "sets the tone" - the wedding is likely to be very informal and relaxed.
It accepted "gives the tone", very literal, but I woudn't know how to write this expression in English.
It's weird to be used with vestido, it's normally with people:
Você dá o tom = you determine how it's going to be.
It's a musician's expression extended to other fields.
One musician gives the tone/note and the others follow.