Simply adjust /etc/lighttpd/lighttpd.conf and change server.port to whatever you like. DietPi is not using the orriginal PiHole web server configuration as we need to ensure web server flexibility to be able to run other web apps as well. Which is not possible with original PiHole conf. That’s how my system is running for long now. I moved lighttpd to port 8080 and have NPM running on 80, 81 and 443. Yes usually NPM require port 81 as admin interface port.
root@DietPiProd:~# ss -tulpn | grep LISTEN
tcp LISTEN 0 5 127.0.0.1:4711 0.0.0.0:* users:(("pihole-FTL",pid=491,fd=14))
tcp LISTEN 0 1024 0.0.0.0:8080 0.0.0.0:* users:(("lighttpd",pid=10253,fd=4))
tcp LISTEN 0 4096 0.0.0.0:80 0.0.0.0:* users:(("docker-proxy",pid=904,fd=4))
tcp LISTEN 0 4096 0.0.0.0:81 0.0.0.0:* users:(("docker-proxy",pid=885,fd=4))
tcp LISTEN 0 32 0.0.0.0:53 0.0.0.0:* users:(("pihole-FTL",pid=491,fd=9))
tcp LISTEN 0 1000 0.0.0.0:22 0.0.0.0:* users:(("dropbear",pid=327,fd=4))
tcp LISTEN 0 4096 0.0.0.0:443 0.0.0.0:* users:(("docker-proxy",pid=865,fd=4))
tcp LISTEN 0 5 [::1]:4711 [::]:* users:(("pihole-FTL",pid=491,fd=19))
tcp LISTEN 0 1024 [::]:8080 [::]:* users:(("lighttpd",pid=10253,fd=5))
tcp LISTEN 0 4096 [::]:80 [::]:* users:(("docker-proxy",pid=911,fd=4))
tcp LISTEN 0 4096 [::]:81 [::]:* users:(("docker-proxy",pid=891,fd=4))
tcp LISTEN 0 32 [::]:53 [::]:* users:(("pihole-FTL",pid=491,fd=11))
tcp LISTEN 0 1000 [::]:22 [::]:* users:(("dropbear",pid=327,fd=5))
tcp LISTEN 0 4096 [::]:443 [::]:* users:(("docker-proxy",pid=872,fd=4))
root@DietPiProd:~#
root@DietPiProd:~# docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
1983c6e6e54a jc21/nginx-proxy-manager:latest "/init" 9 months ago Up 2 days 0.0.0.0:80-81->80-81/tcp, :::80-81->80-81/tcp, 0.0.0.0:443->443/tcp, :::443->443/tcp npm
root@DietPiProd:~#
Creating a custom conf using server.port := 81 will create an additional port only.