You made me watch again that Express Video Tutorial I published almost 2 years ago. 😉
I gave several ways to customise a site design and "not so" complex HTML is one of them.
All depends of the way you want to work and what you want to achieve. (if your design is complex)
You can do quite a lot using the wiki syntax and it is always available on screen through your Tiki editing front-end.
Also you don’t need to "re" design everything and with the use of a few modules and plugins you can give your site a very specific look.
Where you find limit you can add an HTML plugin to write code.
IMO this has some limits and for heavily or complex project a smarty template is much more simple to manage, test and write.
Knowing php is nice to have to work with smarty template but not a must... HTML and CSS yes.