So with the Power Chart, when changing it to Historical View, an extra chart shows up below the main chart. You can right click it and change which pen this chart shows:
When you are injecting CSS into a style, this is often where the CSS string is placed. This really just convince. The only thing important is that the style class isn't expecting a non-string argument.
I link to one of @victordcq's post in that same that is an explanation of how to accomplish this.
The "supported" way to do this is by adding the CSS into a Theme file but that is not as convenient.