Custom Stylesheets for Individual Blogs

Hello!

So on my network i had a request for a few customizations to my themes that i have in place. The only issue is that i want to be able to make these customizations for this particular client but do not want these reflected on the overall template for futiure and current blogs using this setup.

Is there any plugin, or is anyone aware of a quick call that could be placed into the current functions.php file that would essentially say to check the blogid folder of the respective blog for a custom.css & custom_function.php that would allow further customizations to be made that would not affect the overall theme itself?

I came across this post below but was a bit confused on how to properly implement the idea...
https://premium.wpmudev.org/forums/topic/change-theme-colorsimagery-based-on-blog

Thanks a ton in advance for the help!

Erik