I understand that this probably took you some time so to preface I'm not just trying to pick out things you did wrong, just my 2 cents since it looks like you are just looking to improve!
1. Clickable objects, like the links in the nav bar up top, or the threads/users should have underlined text decoration, or at least some sort of other discernible feature that shows the user they are clickable. Obviously we all know they're clickable because we use the website, but if you were design something new and leave links looking like regular text, it makes for a pretty bad user experience
2. I understand where you're going with the nav bar thing, same thing as windows 11 and a lot of gamers are pushing for game UIs (d2r did this) to become more centered due to newer resolutions/displays or whatever, but I don't really think the way you laid it out makes much sense.
The | way | you | use | these can look a bit weird and I think just having text decoration would be enough to separate each link rather than trying to split them with the | symbol
It also just seems to take up more space vertically for almost no reason, compared to what exists currently. It's kind of like putting this on a resume:
University
Year
Course Code
GPA
Instead of aligning some to the left and some to the right, using the horizontal space better. You do the same thing with the add reply/new topic - I can see that you're trying to put it on the same side of a lot of other information, like the page numbers, but really it just pushes the content down compared to what currently exists
3. The blue text thing is more of a theme, if you're on a lighter theme it doesn't really make much sense. The red is a bit more universal, as far as I can tell from the themes I've tried out
4. Perhaps just personal preference, but I think the pink name and the teal forum gold is just distracting/unimportant.. Your username effects already show up to other users/in the forums, which is all that's important, and the forum gold is bolded - it's not like it's hard to find or anything. But again, maybe just personal preference.
5. The text being bigger is kind of weird. I don't personally feel like it's too small at the moment, but if you did and you want to make it bigger you need to take into account more than just the text within boxes and notice the titles and stuff too. For example, on the main page it looks like you made the actual content of the announcement larger, but the title of the announcement is smaller, which looks really odd as a user. And again, no underline decoration for the "read more" or the title of the announcement can be confusing for users
ALL IN ALL
I'm not saying it looks bad, I do like most of it. However, most of it is the same as what's already going on with jsp so I'm not sure who I'm complimenting with that one 
Either way this is really good practice and like I said I'm sure you put a lot of effort into this. Good work and please take my comments as constructive criticism - and I'm just one person! 
E/ Oh last thing, to view the full sized image it's a lot easier to just right click -> Open image in new tab
This post was edited by Soroush on May 22 2022 12:33pm