The quickest way to customize a Tiki site is to use its built-in Site Identity feature. While not "theme-making" in the full sense, it is mentioned here because it is related and might be used in conjunction with other aspects of site design.
Site Identity feature helps 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.x
Site Identity can be enabled by switching the checkbox on (off by default) at Admin -> (Admin Home ->) Features page. The main sub-feature is the Site Logo which will appear on the top Site Header of every TikiWiki page and by default it links to the index page ("./") of the used site as is common.
Site Identity Settings can be found on Admin -> (Admin Home ->) Site Identity (it used to be on the General settings page).
Currently you can: