- A "Layout" is how text and images are arranged on a slide.
- A "Theme" contains globally predetermined appearances within the deck, i.e. fonts, sizes, colors, images, etc.
A "theme" includes a layout, but you do not have to have a theme to alter any layout.
You can manually alter the layout of any slide to match some other "theme".
You can't "combine" themes because
Themes specifically designate aspects of the deck. One theme will overwrite another theme. You can't tell the slides in the deck to have a red background from
Theme A while at the same time having a blue background from
Theme B for a few slides. Theme settings are global.
Merely create a new master slide and manually alter that slide to anything you want - change it's colors, add a background image, etc.