Blogger has been around a few years, and there are a lot of people who have developed custom templates. Unfortunately, most custom templates available are written for Classic Blogger.
A New Blogger blog, when normal, uses a Layouts template. A Layouts template, with the dynamic HTML and the widgets, requires XML. A Classic template contains only HTML.
To use a template developed for Classic Blogger, you'll first have to revert your blog. Then, you can upload the custom template, and publish your blog.
Be aware of what you'll be losing when you do this.