You’ll find the Themes tab on the far left-hand side, in the Editor. It’s second from the bottom, and its icon changes seasonally. 

PicMonkey themes organize effects, graphics, frames, text, and other photo editing features by a particular theme, such as Sweethearts, Winterland, or Celebrate. Themes are great if you're creating images for a particular holiday, occasion, or season. Some of them are based on a visual style such as comic books (Comic Heroes) or project goals such as back-to-school (School U).

Halloween-related themes (Witches, Demons, Zombies, Vampires) offer photo-realistic costumes, effects, and graphics that allow you to transform your subjects into otherworldly creatures and create a scary mood. These themes also offer informational quick tips, denoted by a light bulb icon, that explain how to use them.

More resources:

Tutorial: Use the Pop Art Photo Effect in PicMonkey

Tutorial: Make Any Photo Festive with Winterland

Tutorial: Zombify Yourself with PicMonkey