I would share with you our config of Xenforo, based on a vBulletin 4.x import in 2015.
So, this is the Forum Home
As you can see is a huge UI.X modified version, lot of edits are in the EXTRA.css (you can download it from Console).
In Forum Home we used a sprite custom icons and used UI.X options to add class and manage better positions of each icon.
The sub-forums are using font-awesome as default UI.X.
In the top you can see Popular Tags, which are generated through a combination of TagEss + XenTag, while in the bottom left you can see a "Luminous Trigger" who set a global opacity from 0.7 to 1.0 for auto-set lightning.
Now go in to a forum:
As you can see the header was changed by a custom library called "Luminous Engine" who detects the Title forum and load a custom graphic. We have at least 50 custom graphics for each forum with their menu (in top right) and some custom links.
You can see also a special widget for take all threads based on a forum, thanks to Widget Framework.
This board generate at least 600k/m visits and this is possible thanks to [bd]Cache + memcached engine (soon with Redus).
Now go in to posts:
Is pretty similar to UI.X, there are some little changes like subtitle with Extra Throphies by ******* and medals who says which level user got.
Signatures are collapsed with a low opacity that goes high with mouse over effect in CSS3 (opacity 0.6 -> 1 with transitions options).
That's all, feel free to write if you have some question about design and plugins choosing.