If I had a parent forum category that had many level 2 and level 3 forum sections

example:

- Forum Category (Parent forum)
-- Level 2 Forum Section
--- Level 3 Forum Section
--- Level 3 Forum Section
--- Level 3 Forum Section
-- Level 2 Forum Section

How can I make it so that all of the forum sections within the forum category have the same style as the forum category without having to go to each forum section and changing the style? I've got 5 forum categories on my forum and one of them has its own custom theme and I want the forum sections within that forum category to follow the same style.