When people say website theme, they generally think of a template, and a theme is a skin that wraps around that template creating many looks to one website backbone.
But that is not what we in the Search Engine Optimization (SEO) business think of. A website theme refers to what the website’s content focus is on, irregardless of the look and design feel to the site.
And a website may have many sub-themes. In fact, it is best to build the main site out of these mini sub themes. This is a good technique to help you create the menu structure of your website, try to group similar content together.
This creates mini hubs of value that Google picks up on one page of like content surrounded by supporting linked content helps reinforce its value in Google’s eyes and increase your chances of ranking for that subject.