Plain jeans will do. More beautiful, can be as old-fashioned as you said. Plain jeans are ordinary blue jeans. If you like, you can choose pants with a little fashion sense, but remember not to be too fashionable and fancy, because they won't match your formal dress. Or plain black slacks (different from orthodox ones! ) Both can be used, and it is forbidden to wear sweatpants and canvas slacks! ! ! Shoes can be paired with casual leather shoes. The lace-up looks better, the brown color is good, so don't mention the black one. Clowns!

I probably found a picture, something like this.