Never heard of it, but on browsing their site I saw:
The Road to Mastery
Our tutorials are like typing “become a web development ninja” into Google Maps and clicking “Get Directions”
And I think I might look into it a bit more!
If it’s anything like Wordpress/Drupal/Joomla I’d just install it and have a good play around. You might find it’s the same deal as the above in that you grab an existing template which is near what you need layout wise and then alter it as need be and change the look and feel.
I guess from what knowledge I have with other CMS's that it really depends on what you need to change.
Example, if a particular theme gives you the content you require but not in the style or format you require, the area of change will be primarily the CSS files associated to the pages you need to change.
If however you need to do more than change the colour\text\layout like adding more links or a new DIV etc you will need to start working with the HTML and in this CMS's case maybe the XSLT (but I know nothing of XSLT and its function with this CMS)
The first challenge I guess is working out what files (CSS\HTML\XSLT) connect to each other and what info they hold, make a change, view that change etc.
Install it on a test server and go through the tutorials, this is really the only place you can start IMO
Just been looking at the comparison between WordPress and Symphony at Compare Content Management Systems - cmsmatrix.org - The Content Management Comparison Tool and this gives a good overview of the differences and apart from the XSLT, designing a skin\template will be very similar in both app's.
The matrix also shows that there is commercial support and training with Symphony so is the client prepared to pay for this?
If the job is to big or not within your skill range and the client doesn't want to pay for you to learn, can you not take the job, pass it on to a developer that has the experience and make a middle man cut?