Purchasing a theme for my blogger

HI Dear I want you just to help me is that yet I am hosting my site on Blogspot (https://onlupdates.blogspot.com) while its domain is yet free but I will buy one later after I make everything ready so then use it officially later but I want to buy a good Seo supported.User-friendly navigation and lightweight theme but I don’t know that how can I use it later but because your platform provides the support I can trust you please help me finding these kinds of theme and tell me that will I be able to use it on my blogger hosted site.