What is the ruling on a man imitating a woman in matters that are not specific to her and do not distinguish her, such as buying a mobile phone holder or wearing the color black with the intention of imitation? And how does one repent from that?

Imitating women by men is prohibited in matters specific to women. What is not specific to them does not fall under the prohibited imitation. So, if someone buys a mobile phone holder that resembles the color of his wife's holder, or wears something that matches the color of her clothing, from items not exclusive to women, this is not considered a forbidden imitation.