I've been using the KOMA-Script classes for my writing because of the
flexibility they offer. The current document uses the KOMA-Script book class
which has deprcated fonts incompatible with the fncychap package:
Class scrbook Warning: Usage of package `fancyhdr' together
(scrbook) with a KOMA-Script class is not recommended.
(scrbook) I'd suggest to use
(scrbook) package `scrlayer' or `scrlayer-scrpage', because
(scrbook) they support KOMA-Script classes.
(scrbook) With `fancyhdr' several features of class `scrbook'
(scrbook) like options `headsepline', `footsepline' or command
(scrbook) `\MakeMarkcase' and the commands `\setkomafont' and
(scrbook) `\addtokomafont' for the page style elements need
(scrbook) explicite user intervention to work.
(scrbook) Nevertheless, using requested
(scrbook) package `fancyhdr' on input line 15.
The default book classes and the Memoir class don't have an Extratitle
class, and perhaps lack other environments.
I'll find docs for scrlayer and scrlayer-scrpage to understand what they
do and would like your suggestions for a book class that allows other than
the default chapter heading style.
Rich