The theme editor is a feature available for all users of the game with the 1.3 update released simultaneously with the Snowfall DLC. It can be accessed from the tools submenu in the main menu. With the tool, custom map themes can be created and then shared to Steam Workshop.
Base themes and custom map themes
The game has several built in map themes that are called base themes. Any custom map theme made by the users is derived from these. While the editor allows editing many properties of a base theme, not everything can be changed. The biggest limitation currently is the collections of buildings and other assets are available in a base theme.
Maps and saved games can be loaded with any custom theme that shares the base theme with the original. This is done in the new game, load game or load map panels by selecting a map theme to use from the dropdown called Custom map theme.
Creating and sharing a custom map theme
The first step of creating a new map theme is selecting the base theme. This is an essential choice, since the base theme cannot be changed after a theme is created. You can also naturally load an existing theme.
When the editor finishes loading, a special map with varying features will be seen. Most of the work is done using one of the properties panels that can be opened from the bottom of the screen. The effect of many of the options can best be learned by experimentation, especially with bright colors and/or textures.
Saving and sharing the map themes functions very similarly to the other aspects of the game. When a theme is no longer work in progress, the publish button of the save panel should be checked to make it available for usage the game. It can then be shared to Steam Workshop from the content manager as any other custom asset.
One important thing to notice is that the game treats local and published versions of an asset as two completely separate entities. If a map is based on a local asset, it will still be based on it after sharing. The map theme is not embedded in the map asset but needs to be available separately. Unless the user has a copy of the local asset somehow, your custom theme will be unavailable. The game will show a warning if the user tries to do this. Of course the custom theme can also be changed to the shared version using the dropdown in the load map panel afterwards.
However, the suggested workflow is to create the custom map theme, share it, subscribe to it, and only after that start a map that uses the shared version of the asset. Then when the map is shared, the shared custom map theme can be set as a required item for the map in Steam Workshop.
Usage in general
Clicking on a texture in any of the panels opens a texture picker, which lists the available image files in your ThemeTextures folder. For easier access, the folder can be opened using the little folder button.
Clicking on an image loads it and shows a preview. When a texture is selected, the editor will apply it immediately. The texture will then be compressed (and have its mipmaps generated) in the background, which may take something between a fraction of a second and about a minute. Only after the message about compression vanishes, does the result match what will be seen ingame; the missing mipmaps cause noise in the distant areas while the texture is being processed. The buttons for the textures that are still being processed are disabled.
The textures should be square and have a power of two size (eg. 512x512, 1024x1024; the editor will automatically resize the textures otherwise, which may produce suboptimal results).
The texture tiling sliders are nonlinear to make them easier to use. The text fields next to them can be used for more precise control.
Details for some of the less obvious values in the various properties panels are given below.
The diffuse textures used for the terrain and their tiling parameters can be set from the terrain properties panels. It is suggested to learn the relationship of the grass, cliff, ruined and sand textures yourself by experimentation, for example with textures of different bright colors or clear patterns.
To save resources, the normal textures for cliff and sand are combined into a single texture by the game (with each texture using two channels of a four-channel texture). The normal textures themselves can be changed independently, but the result will only be seen after the editor finishes recombining the data. Because of being in a single texture, the tiling of these two textures has to be the same and can be controlled in the Normal tiling section of the panel.
Geometry details checkboxes allow the small decorative meshes on the terrain to be disabled.
The grass color offsets control the color of the grass in various gameplay-related states. For example, let's say the Pollution offset is (0.03, 0.015, 0.040). In a fully polluted area, this amount would be added to the color when rendering the grass, making it slightly more purple (because the values are the biggest for red and blue). For less polluted areas, the amount is interpolated between (0, 0, 0) and the values given. Unfortunately, not all of the effects can be currently previewed in the editor, so this will require some experimentation.
Replacing the foam texture probably also requires some experimentation. The red channel roughly corresponds to the opacity of the foam, while the green channel is mostly used for rendering polluted water.
The free camera tool in the lower right corner of the screen may be useful to get a good angle to look at the sky when experimenting with the values in this panel.
The border pixels of the moon texture should completely transparent, because they get copied all over the sky.
The textures in this panel are used for roads and buildings.
The upward road diffuse texture is the main pavement texture defining the look of the roads. It will be combined with the textures of the road assets themselves, which provide things like lane markings and details. The downward road diffuse is for similar purposes but for downward-pointing surfaces such as the undersides of bridges.
Building base textures are used for the bases of buildings that are visible on uneven ground. Floor diffuse can be seen through the windows of buildings. Light color palette is used for the night lighting colors for the building windows.
Notice that the temperatures in this panel are always set and shown in degrees of Celsius, since that is what the game uses internally.
Disaster properties (Natural Disasters DLC only)
Sliders can be used to set theme specific probabilities of different disasters. The probabilities are proportional and don't affect overall probability of disasters happening (e.g setting all to 1 is the same as setting all to 100). Setting a probability to 0 disables that specific disaster, and setting all probabilities to 0 disable disasters for the theme altogether.