After the newest change of the OMV Gui, everything looked a little bit too big for me as I don't like "wasted" space on my screen (see also this Pull Request). There's a reason why I have a high screen resolution and it is not to fill it with white but to display information So I created a theme-custom.css which I'd like to share with you. Basically I just decreased paddings and some other stuff. As I'm not a CSS specialist I don't know whether everything makes sense, I used Chrome to determine to parameters and then changed it. And some proportions might also not be perfect, but it's fine for me.
Copy it to /var/www/openmediavault/css/theme-custom.css
Maybe it's helpful for someone likewise as well.
And thanks for the possibility to customize the GUI!