You might want to try Argon, then. It's mobile friendly, has a dark mode, and IMO is the best looking OpenWrt theme I've found. However, it's not available in the OpenWrt repositories, so you have to sideload it from a locally downloaded file. For me, that inconvenience (having to do it again each time you update OpenWrt) was reason enough for me to go back to the OpenWrt 2020 theme.
There's also luci-theme-material, which is in the repo, but I haven't used it, so I'm not sure if it has a dark mode or not. I do remember that it's mobile friendly, and based on Android's Material UI.