Concerns the list of items, that is the window where the e-mail is located.
Can someone tell me how I can get more distinguish between read and unread e-mail?
Now there is too little difference in letter thickness.
Menu / Tools / Settings / Appearance / Lists does not give me that option.
Or do I look with my nose?
Maybe try a different Theme (Menu > Tools > Settings > Appearance > Themes).
I think that the new messages are visible enough though in the default Theme. For one thing there is the orange dot, which is not there with read messages.
If the bold is not that obvious, maybe try calibrating your monitor. You can try this monitor calibration.