LibreOffice 5.2 has an experimental feature called Themes. Themes are set of predefined styles that changes the automatic styles. By applying a them into a document the automatic styles are changed and the document has a new look. There is no help page for Themes.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/5332d9c54e70cdc827bad5f2d5d8a42b9e8c0ea7 tdf#105122 entry page for doc themes.
Is this it? Format-Theme missing help. https://help.libreoffice.org/24.2/en-US/text/shared/05/err_html.html?System=UNIX&DbPAR=WRITER&HID=svx/ui/themedialog/___object_1___
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/3cb6dbe25ae87882ca810a56794a05a65412bf17 tdf#105122 Initial contents for document themes help