By “background: none” on the Entities card level you made the whole card transparent - now you see a “view background” color (black, I guess).
The “divider-row” color uses some “card’s color” for a text label: it is not “line on the left, some text, line on the right” - it is “one long line + an overlaying text in a rectangle”.
If you need this “rectangle” to be of some particular color - then you need to style THIS “rectangle” as well.
Here is a link to a PROPER place where you should have started asking card-mod related questions. As for that “rectangle” - check the 1st post in the mentioned thread → link at the bottom → styles for text-divider-row.
I should have pointed you to the main card_mod thread as Ildar mentioned above.
We can finish this thread out with the following code. You cannot make the rectangle transparent because the line will be visible through the lettering. The background color of the rectangle will have to match the the card’s native background color.
background-color: #111111 !important; this is where you would adjust the color to match the card background.