That would be incredibly difficult for an arbitrary WordPress theme, as they're allowed to do a heck of a lot more than just present content from the database i

That would be incredibly difficult for an arbitrary WordPress theme, as they're allowed to do a heck of a lot more than just present content from the database in straightforwardly generated HTML and CSS. Presumably you have a blog template and want to make a small utility or portion of your site match the template visually, right? That is straightforward to do with a bit of elbow grease: create a minimal blog post with a placeholder $TITLE for the title and $POST for the body content, publish it, save the HTML, then grep for $TITLE and $POST and replace with the appropriate magic for your templating language of choice. You're probably done now, or done modulo changing some paths to point from your application's directory to wherever WordPress locates the CS/JS files. This general trick helps make individual pages of a WordPress template into any sort of templating system very, very quickly. I've used variants to e.g. make WordPress and Rails apps pretend to be siblings as opposed to mere housemates. (Note that you should examine your theme's licensing; it's highly likely that this is a consequential choice if you're working with a theme you obtained commercially and then embed that template into an application which is not WordPress.) If you want to duplicate functionality of WordPress like e.g. interfacing with the database, offering a backend, etc... be forewarned that you are embarking upon an ambitious and involved project.