The best practice will be to add this on the template, and place it inside <head></head> tag.
<base href="http://your-modx-domain.com/" />
DeLaHelias , There is no limitation in templating with MODx, other than your knowledge in (X)HTML and CSS.
The reason why I say this, because you can put whatever html design that you want as a new template, and use it for your site template. All the dynamic contents will only replace the MODx tag located on your template. So if you create a new template, put [*content*] tag on the right place on where you want your main content to be displayed on every page correspond to the template. That content tag is just an example, more about some other tags that you can use, you can read it through documentation provided, or you can take an example of the basic template included with MODx. You can see that, there are only a few tags that will be added into a plain ordinary (X)HTML layout.
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