string themeName = "Office";
ThemeFontScheme themeFontScheme = PredefinedThemeSchemes.FontSchemes.FirstOrDefault(x => x.Name == themeName);
ThemeColorScheme themeColors = PredefinedThemeSchemes.ColorSchemes.FirstOrDefault(x => x.Name == themeName);
DocumentTheme theme = new DocumentTheme(themeName, themeColors, themeFontScheme);