The Content Editor is the authoring environment used to build and modify parts and features in custom content libraries.
The Content Editor is derived from the Block Editor. Hence settings that control the Block Editor have an impact on the Content Editor as well. For example, if you change the background color of the Block Editor, the background color of the Content Editor also changes.
When in the Content Editor, a special ribbon contextual tab is displayed above the drawing area. When the ribbon is not active, a special toolbar is displayed. Both the ribbon contextual tab and toolbar provide tools to build content. For more information on the things you can do in the Content Editor, see the topic About Refining Content