I have tried one by one but still no luck:
1. The asset serialization mode is already "Force Text".
2. Deleting the whole STM folder and re-import it from Asset Store.
3. Re-apply the shaders. Then I found most shaders works, just the STM UI shader doesn't work. Even STM UI Dropshadow and STM UI Outline work.
I also found the 3D STM element works, but not the UI STM element.
By thw way I forgot to mention that, after importing, the console showed many similar errors of " There Surface shader function 'surf' not found at line xx" in many files. I am using Unity 2018.1.4f1. Hope this info. helps...