Which feature allows users to customize the look and feel of their ArcGIS Enterprise Portal?

The feature that allows users to customize the look and feel of their ArcGIS Enterprise Portal is the portal customization options. This functionality provides a comprehensive set of tools and settings enabling organizations to modify their portal's appearance to better align with their branding and user experience needs. Users can change elements such as the logo, color scheme, and layout, which contribute to creating a tailored interface that resonates with the target audience.

While theme settings can also relate to customizing aspects of the portal's appearance, they are typically a subset of the broader portal customization options, which include all available settings for more holistic changes. Web map templates are focused on how maps are presented and do not affect the overall portal look and feel directly. Service definition files pertain to defining services within ArcGIS and are not related to visual aspects of the portal interface. Thus, the portal customization options encompass a wider range of functionalities specifically aimed at enhancing the user interface and branding of the portal.