Here you can see how to set all available buttons for top toolbar (like Appearance="Full") in HTML Editor component with Appearance="Custom".
If you don't want to use the full buttons set, just copy the text below and remove buttons you don't want to see.

<ed:Editor id="editor" Appearance="custom" runat="server">
   <Buttons>
      <ed:Toggle Name="Bold"/>
      <ed:Toggle Name="Italic"/>
      <ed:Toggle Name="Underline"/>
      <ed:Toggle Name="StrikeThrough"/>
      <ed:Toggle Name="SubScript"/>
      <ed:Toggle Name="SuperScript"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="ForeColor"/>
      <ed:Method Name="ForeColorClear"/>
      <ed:Method Name="BackColor"/>
      <ed:Method Name="BackColorClear"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="ClearStyles"/>
      <ed:HorizontalSeparator/>
      <ed:Select Name="FontName"/>
      <ed:Select Name="FontSize"/>
      <ed:HorizontalSeparator/>
      <ed:VerticalSeparator/>
      <ed:Method Name="Undo"/>
      <ed:Method Name="Redo"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="Cut"/>
      <ed:Method Name="Copy"/>
      <ed:Method Name="Paste"/>
      <ed:Method Name="PasteText"/>
      <ed:Method Name="PasteWord"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="Paragraph"/>
      <ed:Method Name="JustifyLeft"/>
      <ed:Method Name="JustifyCenter"/>
      <ed:Method Name="JustifyRight"/>
      <ed:Method Name="JustifyFull"/>
      <ed:Method Name="RemoveAlignment"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="OrderedList"/>
      <ed:Method Name="BulletedList"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="DecreaseIndent"/>
      <ed:Method Name="IncreaseIndent"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="MakeNotEditable"/>
      <ed:HorizontalSeparator/>
      <ed:VerticalSeparator/>
      <ed:Method Name="CreateLink"/>
      <ed:Method Name="InsertTable"/>
      <ed:Method Name="InsertDiv"/>
      <ed:Method Name="InsertHR"/>
      <ed:Method Name="InsertIMG"/>
      <ed:Method Name="InsertIcon"/>
      <ed:Method Name="InsertSpec"/>
      <ed:Method Name="InsertFlash"/>
      <ed:Method Name="InsertMedia"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="InsertForm"/>
      <ed:Method Name="InsertCheckBox"/>
      <ed:Method Name="InsertRadio"/>
      <ed:Method Name="InsertTextBox"/>
      <ed:Method Name="InsertPassword"/>
      <ed:Method Name="InsertHidden"/>
      <ed:Method Name="InsertButton"/>
      <ed:Method Name="InsertSubmit"/>
      <ed:Method Name="InsertReset"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="ToLowerCase"/>
      <ed:Method Name="ToUpperCase"/>
      <ed:HorizontalSeparator/>
      <ed:VerticalSeparator/>
      <ed:Method Name="GetFormat"/>
      <ed:Method Name="ApplyFormat"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="InsertAnchor"/>
      <ed:Toggle Name="AnchorsOn"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="InsertPlaceHolder"/>
      <ed:Toggle Name="PlaceHoldersOn"/>
      <ed:HorizontalSeparator/>
      <ed:Toggle Name="Ltr"/>
      <ed:Toggle Name="Rtl"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="SelectAll"/>
      <ed:Method Name="SelectNone"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="Print"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="SpellCheck"/>
      <ed:HorizontalSeparator/>
      <ed:Method Name="IncreaseHeight"/>
      <ed:Method Name="DecreaseHeight"/>
      <ed:Method Name="Preview"/>
      <ed:Method Name="ContextMenu"/>
      <ed:TextIndicator/>
   </Buttons>
</ed:Editor>