Inconsistent gray used for energy graph & panels

[arouinfar]

All credit goes to @phet-steele for spotting this one.

While not technically found during phetsims/qa#165, I'm going to go ahead and tag it because it's a change that needs to be made before the next RC.

The Energy Graph and Panel/Toolbox on the right are using different shades of gray. According to Illustrator's color dropper, the Energy Graph is #EEEEEE and the other panels are #E6E6E6.

I prefer the somewhat lighter shade used in the Energy Graph, so @Denz1994 please update the right panel/toolbox backgrounds to match.

[Denz1994]

Wow, what a catch @phet-steele. @arouinfar Can you check master? The color does make the screen feel a bit lighter.

[arouinfar]

Love it!