Pufrobo hover overlay not working

I have tried to use the advice in this post, but the fix does not work for me:
https://wordpress.org/support/topic/pufrobo-hover-doesnt-darken-thumbnail/

Has there been a change in the last 2 years, meaning that a different fix is necessary?

I cannot share my website yet since its password-protected prior to launch on 20th March.