It would be most convenient, if we could configure an URL for a custom style sheet, which would then be included as the last style sheet when serving pages.
Creating a custom theme is too heavy a solution for just tweaking minor issues, not least because a custom theme would not benefit from bugfixes and/or updates in the Zabbix distribution.
As it is, I decided to append a hack to browsers.js instead of changing the theme CSS file itself or modifying any PHP code:
A bit ugly yes, but it works perfectly well and should survive updates nicely.
For curiosity, attached is our current additional style sheet for 3.2.1. It's very much more compact than the default.
This feature request is naturally related at least to
ZBX-10504, "Get back compact 2.x web-interface layout".