Changing settings at runtime

Every module reads its settings asset live, so a script can change values at runtime:

using TheyCallMeAlfred.Fable;
using UnityEngine;

public class GrassAmountOption : MonoBehaviour
{
    [SerializeField] FableSystem rig;

    void Start()
    {
        // Work on a copy, or Play mode edits are saved into the project's asset.
        rig.grassQuality = Instantiate(rig.grassQuality);
    }

    public void SetGrassAmount(float share) => rig.grassQuality.densityScale = share;
}

Settings assets are assets: in the editor, a change made in Play mode stays in the asset after you stop. Swap in a copy made with Instantiate first, as above, the way the Core Demos do.

Most values apply on the next frame. Some only apply when grass is built: anything under Density, Blade, Size, Orientation and Placement, the surfaces, and the wind direction that blades face around. After changing those from a script, call GrassInstancer.RefreshFromSettings() (or BuildNow() when only the blades changed) on each affected patch. FableSystem.GetGrassPatches() lists every patch under the rig.

To drive the time of day from a script, set timeOfDay on the Skybox settings (with animateCycle off). SkyboxModule.CurrentTimeOfDay reports the time actually shown, including while the cycle runs.