No, but I do intend writing some guidance. I'm still a bit new to ISPConfig so don't think I'm quite ready.
But here's some hints to begin with:
For a start, my recommendation is to avoid trying to write one from scratch. Evolve your new theme from the existing default theme.
Certain pages like the login page are hard-coded to use the 'default' theme.
I recommend renaming the default theme to 'original' and then creating a symbolic link from 'default' to original.
Then when you have a theme you'd like to use in place of the default theme, you just need to change the symbolic link. For some sites you may want to select 'original', or your new theme or 'default' which will point to wherever the symbolic link is linked.
To create your new theme, copy the default them (now named 'original'?) to form the base of your new theme. I would also recommend deleting the icons and yaml folders from your new theme and replacing them with symbolic links to the appropriate folders in the original theme. You are unlikely to need to change the contents of those two folders.
so in code...
mv default original
ln -s original default
cp -r original mytheme
rm -rf yaml
rm -rf icons
ln -s ../original/yaml yaml
ln -s ../original/icons icons
When making changes don't forget that Firebug is your friend!
(Lets you practice css changes on a live site).