Much better, although I don’t get the colors. Some minor mysteries makes the page interesting.
But can’t we go with some kind of accordion or a tree and show the latest posts after one of the categories is opened/expanded/clicked? Plus, the filter above with ‘latest’ and ‘top’ could be implemented as a sort like Reddit does. Meaning you don’t see the 5 latest posts on click at the category Lounge, for instance, but the 5 hottest if the respective sort button is toggled on.
The header is still too empty when no topic is selected. I would show OSD in that case. And perhaps keep it as breadcrumb to go home.
Home was a topic and is still; not sure if clicking the logo is really bad but often it is prohibited by guidelines. And users may not be aware this link without any label (would be better with [OSD] next to the icon). The blue box with “Welcome to the Open Source Design forum!” misguides me to another page. I expected the link to stay on this page but never to load another site.