Theme: Twenty Eleven How to use light & dark themes for different posts/pages?

So I’ve seen a lot of posts but most are about different themes. I want to use the same Twenty Eleven theme but the dark option for my graphic design pages and the light option for my blog and all it’s categories. I would also like to use different headers for both.

Is there a plug-in for this? If not could someone please help me understand how to go about it. (I’m kind of scared to do the child theme thing but if I must I will) I’ve just recently started getting to know the backend side of things so please give me the ‘for dummies’ version of things.

If it helps my site is https://www.parasuniversal.com, the header that is showing needs to be just for my blog area.