Board Thread:Support Requests - Designing Your Wiki/@comment-26945995-20161110155321/@comment-9605025-20161112222657

Unfortunately, I don't think you can. You are using portable infoboxes which have the style specified through the CSS already mentioned in the other replies. However, I don't think this provides a way to format a specific cell.

If you want to have the head cell be a different color, you would need to use a classic template (see Help:Templates and Help:Classic infoboxes) instead of a portable infobox. In the example you gave, they use several layers of transclusion:
 * Benimaru_Shinmon transcludes Template:Infobox transcludes Infobox:Benimaru_Shinmon transcludes Template:Infobox/Character which is a classic infobox.

The markup they use to get the different color, and consequently the markup you copied, is intended for classic infoboxes not portable infoboxes. While it is conceivable that there may be some ability to create a hybrid, I am not convinced this would be a good solution. As far as I am aware, there was no attempt to make the two different syntaxes interchangeable. As such, I would assume that most methods of creating a hybrid template (if any exist at all), would be unstable.