Wikipedia
Basic dress
A basic dress is a usually dark-colored dress of simple design which can be worn with various accessories to suit different occasions. Different kinds of jewelry, belts, scarves, and jackets can be worn with the basic dress to dress up or down. A little black dress is an example of a basic dress.
Usage examples of "basic dress".
So of course I'm in my Basic Dress Uniform and carrying a side arm.
Expensively attired, his basic dress, was the tailored blue uniform of a vice admiral.
Charlotte came back with simply a hat and a handbag added to her basic dress.