The toolbar buttons in the HTMLEditor control of AJAX Control Toolkit can be customized. For this, create a new server control deriving from HTML Editor and override the FillTopToolbar() method as follows:

public class LiteEditor : Editor
{
    protected override void FillTopToolbar()
    {
        TopToolbar.Buttons.Add(new Bold());
        TopToolbar.Buttons.Add(new Italic());
        TopToolbar.Buttons.Add(new Underline());
        TopToolbar.Buttons.Add(new Paragraph());
        TopToolbar.Buttons.Add(new OrderedList());
        TopToolbar.Buttons.Add(new AjaxControlToolkit.HTMLEditor.ToolbarButton.BulletedList());
        TopToolbar.Buttons.Add(new ForeColor());
        TopToolbar.Buttons.Add(new ForeColorSelector());
        TopToolbar.Buttons.Add(new ForeColorClear());
    }

Bold, Italic are pre-defined buttons available within AJAXControlToolkit.HTMLEditor namespace.