Ah, I see that it was one of the themes available by default during installation, so I didn’t consider that aspect. 
After a little exploration, you’re right. I just unchecked all the third-level categories, and the second-level categories are no longer regular large blocks similar to the first-level categories. Instead, they appear as small blue blocks, just like the category indicators for third-level categories and topics. 
Initially, I even thought that even without third-level categories, the second-level categories should still be the same large blocks as the first-level categories… I misunderstood. 
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