Adding this method to your SysSingleton and marking it with Eco.ExternalLateBound=true can give you access to functions inside the TurnkeyServer:

SysSingleton.MiscSetting(setting:String):Boolean

The recognized values of setting are currently:

Setting	Explanation
resetcss	this will regenerate the CSS from StylesInModel and ThemeAsData on the next read

See also: Theme as data