Yet another "How do I change this color" question

I’ve gotten my interface almost the way I want, but there’s still one outlier.

How can I change the color of the line-number background when I have selected multiple lines, like so:


Thank you for any help! (I’ve made my changes piecemeal — there wouldn’t happen to be any kind of comprehensive guide to using styles.less to tweak my theme, would there?)

This might make it clearer that some text is selected: