The 'background' is called world shaders.
You can look in your data/shaders folder in your toribash directory for shader names.
Then, ingame, type /lws 'shadername'.inc. It only works when you have shaders enabled. For example, /lws meatwad_silver.inc will set the world shader for meatwad_silver in the data/shaders folder.
/lws stands for /loadworldshader