The CSS and JS editor is only available in some templates. Also it is able to edit the JS and CSS files that are embedded through your template. You can view these files in your index.php
(/templates/your_templates/index.php )
That link you have posted is about creating a component (nothing to do with templates), also is impossible to upload CSS and JS files through the media uploader , first of all the files are uploaded by default in /images/ folder, secondly I am not quite sure how is it possible to work.
BUT there might be a tricky solution for your problem. Each component, module or plugin is able to embed its own styles and Javascript files. Take a look at your template's source code, there must be a line like this
<jdoc:include type="head" />
This mentions to the joomla core that there will be embedded the additional files in the head of the HTML document.
My approach would be presented four generic steps
a) Create a folder where your client can upload several CSS and JS files
b) Create a plugin in order to do the uploaded stuff
c) In your plugin implementation add some functionality for editing the files (you can use any jquery's editor plugin to display the code in a popup or even joomla default editor
d)allow user to choose which files wants to embed
As I can imagine it will be a very hard task to accomplish since you have to face lots of problems (file uploading restrictions, file rights etc) but it's not impossible to do
Good luck with your project
