trouble with nth-child and grid and selector

I have a gutenberg core grid. It has 4 columns. I am trying the below, but it is modifying e.g. the left side bar in the backend, and a lot of the right sight bar. So from this i infer the selector is not narrowing it.

How would I go about using the selector to match the sub-blocks in a grid?

selector > :nth-child(8n-0), :nth-child(8n-1),:nth-child(8n-2), :nth-child(8n-3) {
background: #a4f4f4;
}