Most of the criticism surrounding Samsung's software is about the looks more than the functionality, but with the Galaxy S6 and S6 edge Samsung is giving users the ability to quickly and easily change the look of TouchWiz. Usually reserved to those who want to root their phones or use various other tricks and third-party apps, theming is now baked into the software on the latest Samsung flagships. It isn't as complete or tweakable as some may want, but it offers more customization than any previous version of TouchWiz did.
To access themes on the Galaxy S6 you just hit the settings, and find it right at the top with other commonly-used quick launch buttons. The setup is very simple — you can access previously-downloaded themes and apply them, or hop into the theme store to browse for more.
Even though the phones aren't actually available for sale yet, Samsung has commissioned a pretty large number of full-device themes to have in the store from day one. There are movie-themed choices like Avengers as well as cartoon offerings and just artistic ones from names you've heard (but most you haven't), all offering very interesting looks that you can apply to your phone.
When you choose a theme, it's an "all or nothing" situation — themes change your wallpaper, icons (of first-party apps), font, settings menu and every pre-installed Samsung app as well including contacts, the phone dialer, messaging app and more. It goes pretty far overboard when you've gone with a cartoon theme that looks like a child made it (on purpose), but is completely bearable if you go with something more minimalist. The biggest complaint will likely be that you can't apply just the wallpaper, settings, icons or font — and there's no way to remove just a single part of a theme that you would otherwise like. Perhaps Samsung will give you this customization going forward, but right now you're stuck with the choice to use an entire theme.
Samsung also hasn't been clear about how you can upload these themes or whether enthusiasts will have some ability to upload their own complete themes, but that'd be another great step toward supporting customization in its software. Even without those customization options this is still a big deal for folks who have always been rubbed the wrong way by Samsung software — you can quickly change the look of your entire phone with a manufacturer-approved one-tap method.