High-level setting in Confluence, which determines the overall look and feel for all site spaces.
The word theme is used by Atlassian to define a theming add-on, like Theme Press. Out-of-the-box Confluence installs with a default for this setting, which Brikit refers to as the Confluence Default Theme. This setting controls the presentation of all spaces across the site, and can be changed in the General Configuration area of Confluence Administration.