How to implement such logic of selection of elements?

Question

css

Good day. How to highlight the first and last element in each thread? The number of elements can be completely different.
https://jsfiddle.net/c5b0mda1/2/

Answer 1

If you want pure CSS, then you will have to add additional wrappers for the left and right block groups.
If for some reason this is not suitable, then a JS script.