You can edit any aspect of a template, so feel free to choose one that mostly matches your vision and tweak it as you see fit.
You can apply themes to your project at a later time after you've added content. Click the Design tab and select a theme. It will be immediately applied to your project. You can either undo it (Ctrl + Z) or revert to a blank theme if you don't like it.
You can access templates from the File tab. Click New and then browse the available templates. You can also download additional templates from a variety of resources online.