Theatrical lighting often makes the backstage area very warm so try to wear clothing that is both comfortable and cool. Shorts are not usually worn by costumers during a show, but check with your supervisor as they may be permissible during the work call or if the temperature is extreme.
Try to organize several sets of blacks; six sets of tops and trousers should get you through a week of 8 shows over six days. This way, you wont have to do laundry every night after you get home from the show or worry about what you will wear every day. I usually start out in a short sleeved top but bring along a long sleeved sweater or fleece, to cover my arms or if I feel a chill. Second-hand clothing stores usually have a large selection of styles of black jeans, trousers, t-shirts and tops, and the prices are fair. You can look good for work every day, without spending a fortune.
I have developed quite a collection of blacks over the years, in a few sizes; jeans, t-shirts, wool socks, cotton socks, polyester trousers, short sleeved blouses, long sleeved tops and black shoes. They are my work clothes, my uniform.
Wear black socks with your black footwear. Try to buy six pairs of black socks and when they are dirty, safety pin the socks together so you wont lose one in the washer. If you have the perfect pair of comfortable shoes but they are not black, try to dye them. Shoes need to be comfortable and provide support; costumers often carry large costumes and laundry baskets backstage. Soft soled shoes keep the noise down when walking backstage. Theatres are designated as industrial workplaces so open-toe shoes or sandals are not worn to work.
Dark laundry needs special care. Empty the pockets and turn jeans and trousers inside out before washing; turn right side out once they are dry. Wash in cold or cool water as hot water speeds the fading process and use a mild detergent or one specifically designed for dark clothing.
Stage shows are timed; they start at a set time, break for intermission at a set time and end roughly at the same time each night. Costume changes may be set at specific times during the show and you may need to track the time with an old fashioned wrist watch or stop watch ... Am I dating myself again?