Im Winter trage ich zu meinem schwarzen Rock auch eine schwarze Jacke.
What is the meaning of zu here? Why is Rock dative while Jacke is accusative?
You also asked about the cases.
This is your sentence structure:
[PP: Im Winter] [V: trage] [NP: ich] [PP: zu meinem schwarzen Rock] auch [NP: eine schwarze Jacke].
Rock must be dative because it is part of a prepositional phrase with zu, and the preposition zu demands dative. Zu wem trage ich die Jacke? Zu meinem Rock.
Jacke must be accusative because it is the direct object of the verb tragen. Wen oder was trage ich? Ich trage die Jacke.
Zu in this context means that you are combining Rock and Jacke. There is an emphasis that you picked the Rock first and then added Jacke to match a certain style. It is often used when it comes to fashion accessory but also when adding e.g. a certain wine to a meal.