Back to the subject at hand. If anyone is interested in using a tool that bootstraps creating a custom manager there, this may be helpful:
http://devries.jp/yomonger
Yomonger is a Yeoman Generator that automates the build of a custom MODX Manager theme. You'll be asked things like what to call it, if you'd like to use a CSS preprocessor such as Sass, if you'd like to start off with the default layout .tpl files and images, if you'd like to include the source Sass styles of the default theme, and some author related information.
You'll then have a custom them scaffolded to be much like the default theme. That means it will be set up as a grunt project and able to use the various Grunt Commands.
https://github.com/modxcms/revolution/tree/develop/_build/templates/default#grunt-commands
You should first be fairly familiar with the tools mentioned in the Contribution Guide of for the MODX core themes.
https://github.com/modxcms/revolution/tree/develop/_build/templates/default
You'll need to have Node.js and it may be worth following the Installation instructions mentioned above.