What i've done is screen shot captured the overall theme for the core, the tiles, door frames, and the worker skins. For workers without a dungeon theme, they are in a collection together.

My personal favorite, this theme is unlocked by completing all Bonus My Pet Dungeon achievements.

Originally this was unlocked by those who had the game during early access, but now it is unlocked by completing each Undergames Level on the Master difficulty. They do not have any special worker skin, and use Oberon's workers.

Unlocking this theme is accomplished by reaching 150,000 points in all My Pet Dungeon levels, other than Deepwater, which simply requires 100,000 points.

This theme is unlocked by completing all the My Pet Dungeon levels.

This theme is unlocked by completing the 4 secret achievements of the Heart of Gold campaign.

Everyone starts with this.

This Theme is available to those who buy the Underlord Edition of the game. You're also capable of upgrading if you did not originally purchase the Underlord Edition.

This theme is unlocked by just completeing every level of the Original Campaign.

This Theme is available to those who have completed all the non-timed Bonus Objectives in the Original Campaign.

For the gold version, you simply need to beat every level of the Heart of Gold Campaign
The silver version is for completing Heart of Gold Bonus Objectives

You get this theme for beating all the Underlord personalities on the hard difficulty in 1v1 Skirmishes

This theme was available for a certain goal amount of donations for the game during it's Kickstarter Campaign. It is now available along with the Cynical Imp in the Kickstarter package.

You must win every Undergame level as Shale for this theme.

You must win every Undergame level as Volta for this theme.

You must win every Undergame level as Lamash for this theme.

This theme was available for those who reached Founder level during the game's Kickstarter campaign. It is now available for purchase along with thee Cynical Imp and Kickstarter Theme for the Founder's package.

Blazing Workers: Complete all Speed-run Achievements for the main Campaign.
Legendary Workers: Reach wave 75 in Crucible
Heroic Workers: Reach wave 50 in Crucible
Champion Workers: Reach wave 25 in Crucible
The Cynical Imp, and Cynical Imp (Classic): Available as charity DLC, also packagable with the Kickstarter theme, and the Founder's theme.
Dragon Workers: This is simply unlocked by the achievement that triggers when you play the game during Chinese New Year.
Bunny, Aloha, Jack O', and Santa workers: These are all paid for DLC skins.
Gold Worker: This skin is Given out by the devs for certain acts, such as tournament play or featured maps. The developers are searching for other ways to give this skin out.
Silver Worker: This one is not pictured, but is basically a silver version of the gold workers skin. Currently this is only available to Devs, and there is no real indication if it will be unlockable for players or not.
Yogscast Worker: This Given out at the christmas Humble Bundle last year (2017). I believe the devs are looking for a way to release it.
Prison Architect Worker: You only get this if you have Prison Architect in your game library along with War for the Overworld.