I get a stackoverflow exception on following commented line.
public partial class Settings : Form
Color def_bgdColor = SystemColors.Control;
Color def_fontColor = Color.Black; // ERROR
static FontFamily fontFamily = FontFamily.GenericSansSerif;
Font def_fontType = new Font(fontFamily, 8.0f);
A StackOverflowException is usually caused by recursive function calls. Try taking a look at the method(s) that instantiate a new instance of the Settings class (or post them here). There is probably something that calls the method again, or another method that ends up calling the method.
If your IDE shows gives you a call stack, post that too. I'm pretty sure Visual Studio won't, and since .NET 2.0, you can't catch it.