Bluerender is based on the Sunflow rendering system which uses a scene file to control the way an image is rendered. Bluerender comes with a default scene file (scene.sc, found in your Bluerender install directory) that is a great starting point.
You can modify the default scene.sc file to incorporate changes that you want to apply to all your models, but often you want to make a small change that applies to only one model and the best way to do that is with a custom scene file.
The procedure is actually very simple! Copy the default scene.sc file from the Bluerender installation directory to the directory containing the LXF file and rename it to have the same filename. For example, if your input file is MyModel.lxf then copy the default file to MyModel.sc in the same folder. This naming convention lets you have a different custom scene file for each model even if they are in the same directory.
Now when you start up Bluerender and select your input file, Bluerender will see the corresponding custom scene file and use it instead.
For your convenience, you can also click on the path for the scene file and it will automatically be opened for editing using notepad!