When I create another Style, it is saved in an .xml format file that is ID’s with the Report name… like an .ini file. That isolates it from the other Reports. (In this case, the addon is a derivative of a built-in. But the style created for the addon isn’t reusable with the built-in.) This also means the file isn’t editable in any of the wysiwyg CSS editing tools.
I was expecting that styles would be saved in .css format in the CSS folder of the Gramps User Directory. Instead the det_descendant_report.xml file was in the top level of directory.
<stylelist>
<sheet name="NGSQ draft">
<style name="DDR-ChildList">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The style used for the text related to the children." rmargin="0.000" lmargin="2.250" first="-0.750" tmargin="0.000" bmargin="0.000" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-ChildTitle">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The style used for the children list title." rmargin="0.000" lmargin="1.500" first="0.000" tmargin="0.000" bmargin="0.000" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-Entry">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The basic style used for the text display." rmargin="0.000" lmargin="1.500" first="0.000" tmargin="0.120" bmargin="0.120" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-First-Entry">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The style used for first level headings." rmargin="0.000" lmargin="1.500" first="-1.500" tmargin="0.120" bmargin="0.120" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-Generation">
<font face="1" size="14" italic="1" bold="0" underline="0" color="#000000"/>
<para description="The style used for the generation header." rmargin="0.000" lmargin="0.000" first="0.000" tmargin="0.250" bmargin="0.250" pad="0.000" bgcolor="#ffffff" level="2" align="0" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-MoreDetails">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The style used for details." rmargin="0.000" lmargin="1.500" first="0.000" tmargin="0.250" bmargin="0.250" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-MoreHeader">
<font face="1" size="12" italic="0" bold="1" underline="0" color="#000000"/>
<para description="The style used for second level headings." rmargin="0.000" lmargin="1.500" first="0.000" tmargin="0.250" bmargin="0.250" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-NoteHeader">
<font face="1" size="12" italic="0" bold="1" underline="0" color="#000000"/>
<para description="The style used for the note header." rmargin="0.000" lmargin="1.500" first="0.000" tmargin="0.250" bmargin="0.250" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="DDR-Title">
<font face="1" size="16" italic="0" bold="1" underline="0" color="#000000"/>
<para description="The style used for the title." rmargin="0.000" lmargin="0.000" first="0.000" tmargin="0.250" bmargin="0.250" pad="0.000" bgcolor="#ffffff" level="1" align="0" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="Endnotes-Header">
<font face="1" size="14" italic="1" bold="0" underline="0" color="#000000"/>
<para description="The style used for the generation header." rmargin="0.000" lmargin="0.000" first="0.000" tmargin="0.200" bmargin="0.200" pad="0.000" bgcolor="#ffffff" level="2" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="Endnotes-Ref">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The basic style used for the endnotes reference display." rmargin="0.000" lmargin="1.900" first="-0.900" tmargin="0.200" bmargin="0.000" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="Endnotes-Ref-Notes">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The basic style used for the endnotes reference notes display." rmargin="0.000" lmargin="1.900" first="0.000" tmargin="0.200" bmargin="0.000" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="Endnotes-Source">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The basic style used for the endnotes source display." rmargin="0.000" lmargin="0.750" first="-0.750" tmargin="0.200" bmargin="0.000" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
<style name="Endnotes-Source-Notes">
<font face="1" size="12" italic="0" bold="0" underline="0" color="#000000"/>
<para description="The basic style used for the endnotes notes display." rmargin="0.000" lmargin="0.750" first="0.000" tmargin="0.200" bmargin="0.000" pad="0.000" bgcolor="#ffffff" level="0" align="1" tborder="0" lborder="0" rborder="0" bborder="0"/>
</style>
</sheet>
</stylelist>