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.
Confluence Default Theme vs. Theme Press Theme
Brikit uses the word theme differently, to define an overall branded design created with the Theme Designer, and deployed within the Theme Press environment.