A significant part of the look for each theme is achieved through color grading. Color grading is the process of replacing one color by another one. Cities: Skylines uses a 3D lookup table (LUT) to create a unique look. It is possible to override the built-in LUTs in the Graphics options as well as create a user-made one.
Creating a color correction LUT
The color correction user folder differs in location across the various supported operating systems, but are fairly similar across all of them. The various folder structures are listed below:
/Users/<username>/Library/Application Support/Colossal Order/Cities_Skylines/Addons/ColorCorrections
By placing a 2D representation of the 3D lookup texture in that folder, the game will import and convert it after which it will be possible to select it from the Graphics Options "Color correction override".
The 2D representation needs to be laid out as if it unwrapped a 3D texture. So a 16x16x16 lookup texture (size used for the default LUTs), you should end up with a 256x16 2D texture.
By using an graphics editor (i.e. Adobe Photoshop), the user can for example enhance the contrast, save and import. The game will then render with enhanced contrast. Once a LUT is imported and selected in-game, modifications to the source image are reloaded automatically, so it is possible to fine tweak a LUT while having a live preview of the result in the game.