Need to update theme_overview image

[jmooring]

File: static/img/screenshots/themes/theme_overview.png

Appears here: https://d2lang.com/tour/themes

Issues:

1. Names should reflect changes made in code: consistent theme name casing d2#2322
2. Add "Origami" to the list
3. The actual theme name is "Neutral Grey" (with an e) not "Neutral Gray" (with an a). Either correct the image, or alias "Neutral Gray" to "Neutral Grey" to provide consistent naming with "Terminal Grayscale" (with an a).