Would like to know how can i customize my discourse forum to make categories page look like this. I feel this is better than having a linear display of all categories. My amateurish developer side is also curious to learn some hacks to the amazing discourse forum 
That is an extremely old version of Discourse. We do not recommend that two column layout.
I accept that there must be a very good reason behind discarding such an approach. But for the sake of individual preference, is it possible in the new discourse to revert back to the old look ? i have a discourse forum hosted for trial and am playing around for a while till i get it up and running for educational services
No you can not revert
You could create a custom layout, but its a very complex extension