I am just about to put in a PR for four new Leo themes. I have disliked all the standard themes that come with Leo, and I have seen some remarks on the various discussion threads that suggest I'm not alone. Generally I find the colors to contrasty, the dark themes backgrounds way too dark, and the so-called "Solarized" themes do not have much of an actual Solarized flavor.
If you are not used to solarize themes you may find them weird (I did). I found that they have grown on me over time, and I use one or another most of the time. I do sometimes switch from light to dark depending on lighting conditions (light during brighter daytime conditions, dark at night).
Two of these new themes use the true Solarized colors. Fortunately, @ekr adopted the color naming scheme of Solarized themes even if he changed the colors. This made it fairly easy to make the change. The new solarized light and dark themes use the actual Solarized colors (well, the log pane is intentionally darker, which I like, and some accent colors may not exactly be the same).
A third theme is the same as the solarized light except I changed the background color to a shade of gray - I sometimes don't like the true solarized background color. But the new color has the same intensity as the true solarized background, which is a fundamental principal of solarization. This one is called "tbp_light_quasi_solarized".
The fourth theme is called "tbp_dark". It is not solarized, but the contrast between background and foreground had been reduced. Overall, it seems (to me, anyway) to be much easier on the eyes than any of the standard Leo dark themes.
Oh, yes, the themes use the font "Cousine" that we recently discovered as the preferred font. If you don't install it, then the themes will use the same other fonts as standard Leo themes. You might find the Cousine font size a little large. I found that 10pt was a bit too small for me on my system, so I set them to 11 pt. Cousine seems to be a bit different in size than the others, so you might want to edit the font size.
I have attached screen shots of all the themes on my computer. I didn't attach the themes themselves because I'm about to submit the PR. If that gets delayed, I will post them on this thread.