As you might have noticed, the CSS edits on its own do not completely darken Proxmox's Web UI.
HOWEVER, I have found a way to change that - by modifying it's javascript files.
These modifications are server-side only and currently it is only working on PVE 5.4-3 as I dont have servers that run an older version. If you're running an older version you could send me the files needed so I could take a look at them but for now this is all I have.
Download the bash file for your version and run it anywhere on your Proxmox **node**. If the changes arent appearing on your browser right away, please clear cache.