Add default styling for waterfall chart to chart template

[huong-li-nguyen]

Context: #786

[sammitako]

I can work on this one too, after this GHC week :)

[huong-li-nguyen]

Thanks, @sammitako for offering!

I'll let you know because this has to go through design first. We need to provide a waterfall chart that looks great on both the light and dark themes, so some color testing has to be done first :)

[sammitako]

Sure, thank you for letting me know the process :)