"Themes" are very popular at the moment. In case you've missed them, they're templates that sit above your chosen class, and grant bonus powers throughout the heroic tier. Each theme also has optional Utility powers you can swap out with those from your main class.
In many respects, they're a lot like a Paragon Path, or an Epic Destiny. Themes first surfaced in the Dark Sun Campaign Guide, and thanks to their popularity, Wizards soon released a set for general play through DDI. Most recently, the Neverwinter Campaign Guide featured a bunch of themes for the Forgotten Realms. So, it looks like they're here to stay.
Themes are great for bringing out the unique aspects of your campaign world. For example, if you're playing in Forgotten Realms, your mage could choose to be a Renegade Red Wizard. Or, your entire party could choose to be "Harpers": members of a secret society sworn to fighting evil.
For my rebooted campaign world, making some unique themes seemed a great way to flavour the party. Recently I've been working closely with my players to flesh out the themes they'd like to play, and over the next few weeks, I'll be previewing them here. Stacey's "Bombardier" is the first: a regimental soldier schooled in the use of firearms. This theme works alongside my custom rules for firearms, which I detailed in the recent Gears and Gunpowder article.