Use a float variable (Custom option type) for this.
Since you have a custom script, you can simply register to the global variable handler to get notified of changes and change the setting in that case.
Add this to your class:
protected void OnEnable()
{
ORK.Game.Variables.Changed += this.VariablesChanged;
}
protected void OnDisable()
{
ORK.Game.Variables.Changed -= this.VariablesChanged;
}
protected void VariablesChanged()
{
this.SetQuality((int)ORK.Game.Variables.GetFloat("qualitySetting"));
}
"qualitySetting" would be the variable key, so you can change to to whatever you want to use.