[KevinH]

It is all platform agnostic code. It works in my macOS.

This is strange.

HTMLResource *Book::CreateEmptyHTMLFile(const QString &folderpath)
{
    HTMLResource *html_resource = CreateNewHTMLFile(folderpath);
    QString version = html_resource->GetEpubVersion();
    QString data;
    QString template_path;
    if (version.startsWith('2')) {
        template_path = Utility::DefinePrefsDir() + "/" + "user-template2.xhtml";
        data = EMPTY_HTML_FILE;
    } else {
        template_path = Utility::DefinePrefsDir() + "/" + "user-template3.xhtml";
        data = EMPTY_HTML5_FILE;
    }
    if (QFile::exists(template_path)) {
        data = CleanSource::Mend(Utility::ReadUnicodeTextFile(template_path), version);
    }
    html_resource->SetText(data);
    SetModified(true);
    return html_resource;
}

and

CSSResource *Book::CreateEmptyCSSFile(const QString &folderpath)
{
    TempFolder tempfolder;
    QString fullfilepath = tempfolder.GetPath() + "/" + m_Mainfolder->GetUniqueFilenameVersion(FIRST_CSS_NAME);
    Utility::WriteUnicodeTextFile("", fullfilepath);
    Resource * resource = m_Mainfolder->AddContentFileToFolder(fullfilepath,
                                                               true,
                                                               "text/css",
                                                               QString(),
                                                               folderpath);
    CSSResource *css_resource = qobject_cast<CSSResource *>(resource);
    QString version = css_resource->GetEpubVersion();
    QString data = "";
    QString template_path;
    if (version.startsWith('2')) {
        template_path = Utility::DefinePrefsDir() + "/" + "user-template2.css";
    } else {
        template_path = Utility::DefinePrefsDir() + "/" + "user-template3.css";
    }
    if (QFile::exists(template_path)) {
        data = Utility::ReadUnicodeTextFile(template_path);
    }
    css_resource->SetText(data);
    SetModified(true);
    return css_resource;
}

So long as you used the correct file names it should work.

For epub2:

user-template2.xhtml
user-template2.css

and for epub3:

user-template3.xhtml
user-template3.css