MGTextBox textBox = ...;
MGTextBlock textBlockComponent = textBox.TextBlockComponent.Element;
bool success = textBlockComponent.TrySetFont("familyname", fontSize);

MGDesktop desktop = ...;
FontManager fm = desktop.FontManager;
fm.AddFontSet(new FontSet(...));

MGComboBox<double> comboBox = ...; // suppose we had a combobox whose ItemsSource is a collection of doubles
comboBox.DropdownItemTemplate = (item) =>
{
    string text = item.ToString("0.00"); // convert the decimal to a string with 2 decimal places
    MGButton button = comboBox.CreateDefaultDropdownButton(); // create the button that will host this combobox item

    //  Create the content of the button and customize its font family etc
    MGTextBlock textContent = new MGTextBlock(comboBox.ParentWindow, text);
    bool success = textContent.TrySetFont("fontFamily", 16);
    button.SetContent(textContent);

    return button;
};

MGDesktop desktop = ...;
MGTheme theme = desktop.Theme; //  Note: If the `MGWindow` specifies a theme, that theme would take priority. See also: MGElement.GetTheme()
theme.FontSettings.DefaultFontFamily = "fontFamilyName...";

MGDesktop desktop = ...;
FontManager fm = desktop.FontManager;
fm.DefaultFontFamily = "fontFamilyName...";