I have two sites, using the same theme with a similar layout/appearance and images, but with entirely different content. Would Google consider this duplicate content?
The first thing you have to know is that Google uses your HTML DOM (document object model) to determine where content begins and ends. It can do this fairly well with one page, but uses more than one to know exactly what HTML fragments are header, footer, sidebar, etc. and removes these sections for content analysis. This means that templating is taken completely out of the equation.
From there, Google uses semantics to evaluate the content. Duplicate content is not measured in a linear fashion. Semantic score similarities are used to determine duplicate content. This evaluation allows Google to determine if content was simply reorganized to avoid duplicate content detection. Any two pages of content does not have to be duplicate anymore, it just has to be similar enough to become duplicate content.
Even though the layout is exactly the same across all pages, that accounts for a small percent of the entire source code of any page. If you have tons of unique content per page, then the layout code would make up an even smaller percentage of duplicate content.
However, If your layout consists of a side panel with almost the same text for every page, then the percentage of duplicate content rises.
Just to be safe, compare any two pages using the same basic layout with any duplicate content checker online to see how duplicate the two pages are.
And if your template contains just formatting and no text, then you should use an seo tool like seositecheckup.com to see the code to text ratio. The higher the percentage the better, because then that means theres more readable text in comparison to code when viewing the source code, which in turn causes people to wait less time for the page to load.