Change line color background

I'm using a custom color scheme (that I think I modified from Monokai). I'd like the line number in my scheme to have more contrast to the black editor, similar to what it looks like in the Darcula theme.

I understand that one can change the appearance of the line number (https://intellij-support.jetbrains.com/hc/en-us/community/posts/115000062024-Line-numbers-are-grey-Is-this-configurable-in-any-way-), but it seems like you can't change the background of it. I've tried clicking on the background tickbox but it did not allow me to tick.

Therefore, short of copying the Darcula scheme and modify it to match my current scheme, how can I change the background of the line number?

If that's not possible, how can I quickly "import" my current scheme (fonts, colors, etc.) to the Darcula scheme so I can have both my current settings and Dracula's line number color.