As described in the Preferences section of the documentation:
You can configure a custom palette theme color style with:
defaults write com.stairways.keyboardmaestro.engine CustomPaletteTheme "0,90,74, 110,90,74, 256,90,74"
The style includes 3 or 6 HSB colors (being background, text, highlighted letter, selected background, selected text, selected highlight letter) each specified as hue,saturation,brightness. The background brightness is dropped by 33%, but if there are only three colors, then the selected background color is the background without the 33% drop. You can select the Custom style in the Theme Editor.
If you design a nice looking one, send the values to me and a suggestion for the name if desired and I will consider adding it for future versions (this is where Gemmel Purple came from for example).