Set background color

[SoftologyPro]

How can I change or set the background color? When I render to an EXR file the background is black and when I open the XML in the mtsgui.exe the background is white. How do I set the background to any other color?

Here is the simplest example scene I could create. How do I set the background color behind the single yellow cube?

<?xml version="1.0" encoding="utf-8"?>
<scene version="0.5.0">

<sensor type="perspective">
<transform name="toWorld">
<lookat origin="0,0,25" target="0,1,0" up="0, -1, 0"/>
<rotate x="1" angle="25"/>
<rotate y="1" angle="50"/>
<rotate z="1" angle="180"/>
</transform>
<float name="fov" value="35"/>
<string name="fovAxis" value="y"/>

<film type="hdrfilm">
<boolean name="banner" value="false"/>
<integer name="width" value="640"/>
<integer name="height" value="360"/>
<rfilter type="gaussian"/>
</film>
</sensor>

<emitter type="constant">
</emitter>

<shape type="cube">
<transform name="toWorld">
<scale x="0.5"/>
<scale y="0.5"/>
<scale z="0.5"/>
<translate x="-5"/>
<translate y="-5"/>
<translate z="-5"/>
</transform>
<bsdf type="diffuse">
<srgb name="reflectance" value="#FEFE00"/>
</bsdf>
</shape>

<integrator type="direct">
<boolean name="hideEmitters" value="true"/>
<integer name="shadingSamples" value="15"/>
</integrator>

</scene>