The cubeRenderer bounding box is using the simple line shader in OpenGL, but, unfortunately, is not open to be edited by the public at this time.
The cubeRenderer.renderHighlightedDepthWithCameraPose is also not open to the public to be edited at this time.
You could write your own functions that do the shaders similarly to what is shown here, but this is not an easy task. You should definitely take some time to research a little more about how OpenGL shaders work before trying to implement this:
As for the shaders used when the Mesh is being rendered, you should look into the MeshRenderer.MM and .H files for clues on how the Mesh is rendered. More specifically, the MeshRenderer.MM files uses the LightedGrayShader class found in the CustomShader.h files. If you want to change the way the MeshRenderer.mm file renders the mesh, you'll have to change it there, or write your own shader inside the CustomerShader.h file and call it when the LightedGreyShader class is called by the MeshRenderer Object.
What is it that you are specifically trying to change with the cubeRenderer and meshRenderer shaders?