So after few tinkering attempt. . .

The [theme] field in {{cevio character}} determine the title and heading bg color

But the size and font excluded from that parameter and set to general in the application

Just some suggestion

Extend which infobox element the [theme] modify, maybe?

You could make it more personalized for each character that way if thats what you want

Only if you want to distinct each individual infobox element you may go this way

Otherwise better stick to it because adjoining class will cluttering your sheet

P.s.: the dropdown menu in your wiki is black background in black font in case you didn't notice

I am not sure if this is what Than-Gee-Door was getting at but here is my suggestion. Change the selector for the general infobox CSS to exclude a particular theme/name. Then you can use that theme/name for infoboxes you don't want the general styling to apply to.