The quickest way to customize a Tiki site is to use its built-in "Look and Feel" features. While not "theme-making" in the full sense, this is described here because it is related and might be used in conjunction with other aspects of site design.
The Look and Feel features help an admin to simplify the customization process of his/her site. Instead of manually hacking the code of the template (*.tpl) files, you can have your own Site Header area with Site Logo and more optional elements set up directly from TikiWiki Admin section. Site identity was introduced in Tiki 1.9, and as of Tiki 3.0 it is no longer an option, but is an integral part of Tiki. Now the "site identity" features are each optional individually.
The various features that contribute to the site identity can be enabled by at the Admin -> (Admin Home ->) Look and Feel page (tiki-admin.php?page=look).
As of Tiki 3.0, you can:
These are the options specifically on the Look and Feel admin page in Tiki 3.0 (though this list is not complete; some more minor items were left out to keep things simple). Other admin pages are also related to visual aspects of the site, such as the Theme Manager (controls application of themes per feature, per page, etc.)
This is the admin interface at "Admin -> (Admin Home ->) Site Identity". (Note that this is an old screenshot and needs to be updated for Tiki 3.0, but you get the idea.)
Please go to:
http://demo.opensourcecms.com/tiki/
user: admin
password: demo
Then go to the Look and Feel admin panel:
http://demo.opensourcecms.com/tiki/tiki-admin.php?page=look