I have the same issue. I have no idea as to any pattern, when it turns up. For me, mostly after I load a save. When setting up the game initially, it usually is fine, but when reloading for the second time, often the fertile texture set (the one you can't change in the settings yet), turns the same neon green which is shown in the info view.

This has happened to me with various themes.

The Mod F:\steam\steamapps\workshop\content\255710\775021554 [ThemeMixer.dll] has caused an error [ModException]

Details:
System.NullReferenceException:
at (wrapper managed-to-native) UnityEngine.Light.set_shadowBias (single) <0x0008d>
at LightingRebalance.UpdateLighting.Postfix () <0x0006f>
at (wrapper dynamic-method) DayNightProperties.UpdateLighting_Patch5 (object) <0x00e9b>
at DayNightProperties.Refresh () <0x000f5>
at ThemeMixer.ThemeMixerManager.set_MoonTexture (ColossalFramework.Packaging.Package/Asset) <0x000c8>
at ThemeMixer.ThemeMixerInfo.LoadAsset (ThemeMixer.ThemeAsset,bool) <0x00562>
at ThemeMixer.ThemeMixerManager.LoadAllAssets (bool) <0x00091>
at ThemeMixer.ThemeMixerManager.Initialize () <0x00225>
at ThemeMixer.ThemeMixerManager.OnLevelLoaded (ICities.LoadMode) <0x00023>
at ThemeMixer.LoadingExtension.OnLevelLoaded (ICities.LoadMode) <0x00067>
at LoadingWrapper.OnLevelLoaded (SimulationManager/UpdateMode) <0x0008b>

Similar bug report here to the op.

Immediately upon loading a map or save with the mod enabled in the Content Manager, but NOT enabled in the Options panel, sometimes the grass texture, or sometimes the sand texture from the theme mixer settings overrides the loading theme texture.

This is easily fixed in game by going to Options, and switching the Mixer on and then off again. But it is useful to know that loading a save or map might have incorrect settings at first.