I am using the dark theme but need more contrast in the messages list regarding which item is selected. I have reviewed the posts about themes and want to customize this part of it. Can you please tell me which line I need to edit for this effect? Thank you!
Maybe your monitor needs to be calibrated. 
In the Dark theme the message list has a grey background, while the selected message is much darker. It is not black but close and is certainly different.
But if you manage to change it, I would also be interested in which line it is.
The contrast is not that distinct because every other message is a different shade of gray, which your screenshot doesn’t appear to have. 
I also have a visual impairment so contrast is super important as is reducing glare, thus my preference for dark schemes.
Ah, a screenshot says it all.
You are not using the default compact view, so the lines are in alternate colors.
Go to Menu > Tools > Settings > Appearance > Lists and untick Use alternating colors.
Now only the selected message will have a darker background.
Hope that helps.
Thank you, it is good to know that is an option. But that makes it harder to distinguish between messages/skim. I’d still like to keep the alternating and change the selected color.