From one of the now deprecated “Client Customization to the Max” Insights sessions:
Var YourNodeValue = Ice.Lib.AppSettingsHandler.GetValue(“YourCustomSysconfigNode”, string.Empty);
YourCustomSysconfigNode is the node in the sysconfig file - I believe it looks in userSettings section first and if not found, appSettings.
Second parameter, in this case stringEmpty, is the Default value to use if the requested node is not found in the Sysconfig.