Changing background color

[JamesMK]

Just add a _2D_background_color shader to your pass shader's output stack

[Rens]

Yep, and if you need it while rendering, e.g. with reflections, you can just plug a Color Share node directly into the pass by selecting the pass in the explorer, opening the render tree, and plugging the color node into the Env Shader Stack port.

[ThE_JacO]

please use the search function in this forum. This question has been asked and answered a number of times already.