FireLance
Legend
Actually, themes give you the option of replacing class powers with theme powers. Most themes also give you an additional power or other additional benefits which do add abilities to your character.4E Themes work a lot like 2E kits; however, instead of adding abilities on top of the character, a 4e theme replaces core powers with theme-based powers.
However, it is entirely possible that in 5e, there will be no additional benefits, and that themes simply present more options. I would be in favor of such an approach, actually, as it would mean you could add as many themes as you want (or leave out themes entirely) without changing the default power level of the characters significantly.