When someone says "Nothing too [X]," it usually means "It will not be good if it is very [X]." In the context of your example, "Nothing too elaborate" means "you do not want an elaborate blouse." (You want a plain blouse, not a fancy one.) Here are a couple more examples: "I'd like to order dessert, but nothing too sweet." (Meaning: I want to order dessert, but I want a dessert that is only a little bit sweet, not too sweet.) "I'm just looking for a basic computer. Nothing too expensive." (Meaning: I don't want a computer that is very expensive.)

"Elaborate" in this context means complicated in design or pattern. "Nothing too elaborate," therefore, means "a shirt that is not too complicated in design or pattern"

It means that you want to buy a one color blouse that has no printing on it(that's what plain means), and that it matches with that skirt.

Here's an example of an elaborate blouse! https://www.dailymail.co.uk/tvshowbiz/article-6152773/Sofia-Richie-dazzles-elaborate-plunging-white-blouse-NYFW.html