There’s not a lot of difference between an Office Theme – a .thmx file – and a PowerPoint template – a .potx file.
Although there are some nuances, the biggest difference is that a theme cannot have slides. Templates and typical PowerPoint files (.pptx, .ppsx, etc.) can have slides.
In fact, a PowerPoint template (.potx) is really just the same as a PowerPoint file (.pptx, .ppsx); the file extensions just tell the operating system how to open them when they’re double-clicked in Finder or Windows Explorer. When double-clicked, a .pptx PowerPoint Presentation file will open in PowerPoint into Normal (editing) View. A .ppsx PowerPoint Show will open in Slide Show View. A .potx will open a new PowerPoint Presentation file that is based on the template. It will be named Presentation 1.pptx or similar. This is to preserve the template so that inadvertent changes aren’t made it it.
You can think of a PowerPoint template or file as being a theme plus slides.
