I'm developing a web page that is customized per client and I'm looking for a NodeJS builder that allows me to configure the CSS and funcionalities of each client.
The module appgen seems to fit the requirements but the problem of this application is that it's still on a early stage of development and probably will suffer major modifications until the first version is released.
imo, this is not a good use for node.js in today's world. Node is about getting down to the metal and custom web application development. For basic customizations and an 'html template' I'd look at something like wordpress, joomla, drupal, etc.
If you'd like to get your hands dirty (and I encourage you to), in the Node world, there are a few web application frameworks you can choose from:
On the cms side, I've heard of this one, but don't know about its quality: