Beyond the realm of theming, there are a number of minor tweaks that integrators can make to a Plone site, and will likely need to make during the course of theming a site, but these tasks do not constitute theming, per se. These configurations enable the theming process, however.
For example, when you add a portlet, the portlet may give you a few options that allow you to control the look and feel of your portlets (such as including borders, including a header or a hyperlink, and so on). If you add a navigation portlet and modify it, or you modify the one that is installed by default, it gives you options to control whether that navigation displays at the root level, whether it shows all parents and children, whether to add borders, provide hyperlinks, and so on. Every portlet is different, of course, and may not provide all of these options.
Additionally, in Site Setup, configuration screens called configlets exist for various pieces of the Plone UI. These...