css-components-src folder contains the necessary tools for creating custom themes.
In order to modify the theme, you can either apply your own custom CSS rules on top Onsen UI defaults (which is good if you just want to change a few things), or create a whole new theme following the steps in the tutorial you posted. I think the latter is not that hard, you just need to install the required tools mentioned there, run the commands and then modify
theme.css file. You’ll see the change sin the previewer and will be able to generate a custom
onsen-css-components.css. After that, just replace the default
onsen-css-components.css with the new one.