There are several ways to accomplish this. The easiest is to paste it as is into a custom theme in a CSS file. Of course this means that it is removed in all cases across all views, so long as that theme is applied.
The next is to inject it into a style. You can learn more about that here:
There are also ways to insure that it is only applied to a specific instance of power chart utilizing a custom style. I'm not too up to speed on that, perhaps @victordcq or @nminchin can provide a more complete solution along those lines.
I also don't believe that injected styles like this work in the designer, but they do work in the browser.