Change white backgrounds of demo SVGs to off-white

[jdm]

According to Magic Leap contacts, bright white stresses the optics of the headset in ways that reduce performance. Changing any demos that involve bright white to an off-white colour instead should make the demo even better.

[jdm]

@asajeffrey Is this something you could address?

[asajeffrey]

Sigh, I guess so.

[pcwalton]

We could also fix this automatically if necessary, by going through the color palette on the Scene and dimming colors as needed.