I'm using the official LibreOffice package for Linux, version 7.0.1, and I've no idea how to change the font for the user interface (dialogs, menu, options, etc). I've been Googling for that for the past two years and nothing has come up unfortunately. It would be great if someone dug deep into the application and resolved this conundrum. It looks like LibreOffice uses some "default" fonts for Windows/Linux/Mac OS but again, it's not clear what the defaults are and how to override them. There's nothing in the options aside from the ability to enable/disable font antialiasing.
Someone who can read code can certainly figure it out but I won't attempt to comb through 1GB of sources: https://github.com/LibreOffice/core
I've found a file which seemingly describes default fonts but it's too much for me: https://github.com/LibreOffice/core/blob/master/officecfg/registry/data/org/openoffice/VCL.xcu