Once upon a time, themes were to be a central part of 5E. You were going to have race, class, background and then multiple themes added onto the PC (one at first, one at 6th, etc...).
At first blush, it looks like they rolled themes into the classes. However, I was wondering if we ever got confirmation that themes were entirely removed as a separate thing. Did we?
Well, the ideas that originally inspired themes are incorporated elsewhere in the system. The evolution is complex.
The original Themes in 4e included some flavor/background element, as well as large mechanical elements like extra powers, extra feats, etc.
During early 5e design, they wanted to simplify the process of choosing skills, and ditched 4e's approach to feats, so they wrapped up all the character customization outside the race/class choice into the "Themes" concept, and split it into two subsystems: Backgrounds, which were packages of skills, and Specialties, which were packages of feats (This second subsystem was originally called "themes" itself, but they realized that term didn't make sense).
So, during the development, the skills didn't change too much (I mean, they expanded and contracted them a few times, but they ended up very close to where they started). Feats, meanwhile, fluctuated a lot, and were redesigned to be fewer, bigger, and optional. Each one was like a mini-specialty. They also put more weight into subclasses, which also filled the role of Specialties. "Specialties" essentially split into two separate systems.
tl;dr: Themes split into Backgrounds and Specialties. Specialties split into feats and subclasses. So backgrounds, feats, and subclasses combined are all 5e's version of the "Themes" concept.
