Now that Envato Forum uses Discourse (correct me if I’m wrong), would that mean in the near future, there will be Discourse theme/skin/plugin?
Not necessarily, no. Both events are mutually exclusive. Interesting idea though! I’m sure someone from our Growth Team will see this and look into it (if they haven’t already). 
Just curious - have you ever skinned a Discourse forum yourself, and if so, how difficult was it?
I am just your typical everyday user which interested in deploying his own website/community. I do currently host a discourse based forum for my friends at work and it’s still basic (no customization due to lack of spare time tinkering with it)
Sadly, I have yet skinned anything. While I do have Exclusive Author badge, I am still learning to become one.
Keep in mind, unlike the more traditional forums, creating themes for Discourse is far more flexible.
You can go beyond just amending CSS, you can amend structure, in a pretty straight forward way.
A classic example is: https://meta.discourse.org/t/sams-personal-minimal-topic-list-design/23552
To see it in action have a look at: