Leo can highlight the current cursor line in the body. If you haven't tried this out yet, there is an item in the
Help/Open Help Topics menu that tells you what settings to use.
The code tries to compute a reasonable highlight color based on the theme in use. You can override this color with your own choice using a setting. But previously, if you opened a theme's outline, say if you wanted to edit it, that theme's color would become used by all open outlines but the highlight code would not change to match. This could produce unreadable text if you were unlucky.
With today's devel branch, the highlighting code notices the change in colors and recomputes the highlight color. The new highlight color will be applied the next time you move the cursor.