Advise Regarding Unbounce Landing Page

Good Morning to all. I am making a Landing Page for an event. I’m currently researching on the best way to do this. I’m thinking of doing a Landing Page on Unbounce. However, I am lost. I have seen an Unbounce template on Themeforest which I like. I want to purchase it, but after I purchase it, how do I customize that template? Would I also need to sign up with and pay in order to be able to customize the Unbounce template which I will purchase from Themeforest? Or is there another way to customize that template without having an Unbounce Account…Any advise would be appreciated.