I switched to the modern UI because the old gray one looks ugly and outdated in the current days. It is not possible to customize the Sencha ExtJS themes that much without buying the whole Sencha tool chain.
For all of you that do not want to use the new theme, i've added support to configure the theme via environment variable in 3.0.76. Add OMV_WEBUI_THEME=(triton|gray) to /etc/default/openmediavault to switch the default theme. After that you need to remove the cache files in /var/cache/openmediavault/cache*.json.
To customize the theme you can also add the file /var/www/openmediavault/css/theme-custom.css with your own CSS code.