do
micro -plugin install monokai-dark
to get an idea of how plugins work.
~/.config/micro/plug/monokai-dark
is the directory of the plugin, just modify the colors in that file and rename to have a custom plugin.
You can use pywal to get example colors from the default wallpaper
wal -i /usr/share/wallpapers/garuda-wallpapers/Ghosts.jpg
The created colorschemes can be found in ~/.cache/wal/, probably it can be used as source for colors 