conf edition-styles.xml

This file is mandatory and takes place in your skin conf/edition-styles.xml
See the page Extending styles to know what to set in that file

If this file is not read, check that you are using org.ametys.web.clientsideelement.styles.SkinHTMLEditorStyleExtension as org.ametys.cms.clientsideelement.styles.HTMLEditorStyleExtensionPoint in the WEB-INF/param/runtime.xml