The game graphics tend to be seen as one thing by people who don't know how they are made. They actually made of two things : the geometry and the textures.
What the textures are
The textures are 2D images that define the appearance of the polygons of the 3D environment.
There are lots of available textures. The number of texture available by default is around 400, and even around 900 if you count all the files eligibles as textures. There also are the textures the game can automatically download from the akimbo database. These last ones are thousands.
In the games using the Cube Engine, such as ACR, there can't be more than 255 texture, plus the sky.
When you load a map, ACR execute the file default_map_settings.cfg then, if it exists, the cfg file of the map.
When you create a new map, the used map settings are the ones of the last loaded map but they aren't saved when you save your map.
The content of the the file default_map_settings.cfg depend on the AC/ACR version you use. There also are mods that have their own version of this file. The version used by ACR 2.6.1, that come from AC 1.2, load 250 textures. If you want to use textues that aren't in this list, you need to create a cfg file for your map.
The texture you can find at browsing ingame the textures list are the already loaded ones.