On her Facebook page, Nina said that Juraj Zigman designed the dress she wore in Amsterdam and that he will also design a dress for the Eurovision performance.
So it seems everyone was wrong assuming Matija Vuica will design her dress.
From eurosong.hrt.hr:
Nina: "Zigman is a fantastic designer and there's a reason why he wins numerous awards and nominations, including the 'Elle Style Awards'. Since winning the Voice, I expressed my wish to meet him and cooperate with him, therefore I'm very glad about this. He is the one of those creative people in their field of work to whom you can absolutely trust and who always makes more than expected."
Zigman: "That work is different from everything else I'm working on, especially commercial collections that are presented on fashion shows. I like the stage and I like when creations can be presented in that way, and each singer I'm working with I see as a challenge, regardless of their music genre. Nina and her song "Lighthouse" inspired me very much. It is rare that the music inspires me that much, but this song took me to another world."