The attached Leo outline contains a script to highlight the line in the body that has the cursor. You can activate it by selecting the top-most node, then pressing CTRL-b. After this, the cursor line should become highlighted anywhere in the outline. Other outlines will not be affected.
If you want to try it out in another outline, copy the script, paste it into the other outline, and run it with CTRL-b.
Activating the highlighting this way is a bit strange, but lets you try it out without needing to modify Leo itself or install a different version. Eventually, the code would be included and activated when the body pane is constructed.
I'd like to know how you like the effect, and whether you think the highlight color works well enough.
Note that the highlight color will be different for different Leo themes (and no-theme color schemes). The script tries to calculate reasonable colors. If the background is light, the highlight will be the same hue but darker. If the background is dark, the highlight will be the same hue but lighter. If the background is very light or very dark, different rules may be applied.
I have tried this out on every theme in Leo's theme directory. The colors seem to work acceptably to me. If you have your own theme that isn't one of the standard ones, I'm especially interested in how that works out. And, of course, post here if you notice any bugs or anomalies.