It depends what you mean about a custom html editor. There are a few possibilities to add HTML, CSS and Javascript code. So if you want to use a editor you can first try out the code in an editor you like and then paste your code in a widget.
Most freedom gives the Rich Text widget. However be awere that changing HTML and/or CSS may affect your standard SitePad code. There is a Embed widget to paste HTML code and furthermore there are widgets that offer adding of CSS code.
You can use the Navigator to goback to your additional HTML and/or CSS code if it doesn't display anything on the screen.