I think this forum uses the Material theme.
The header probably uses versatile banner component. I’m not sure about how they made the “learn more” button. It’s either a component’s included feature or added javascript.
The forum also uses the category icons component. I’ve already used this one once for test purposes.
I see no further modifications at first sight except maybe a bit of CSS tweaks.
So there should be no issue to customize your forum a similar way as long as I have all the design instructions. 