Are there any other Bitcoin and crypto forums, with more ''modern'' themes, that are better or more active than Bitcointalk?
- No.
I am not sure that adding a mobile theme and improving search engine would make a drastic change, but we could use some refresh like big tech websites are doing all the time.
We should also not forget that people die and move on to other things all the time like Satoshi did, and we shouldn't be surprised by this.