On Thursday, 25 June 2020 09:59:46 BST Valmor de Almeida wrote:
> Hello,
> Does anyone know how to fix Thunderbird UI fonts on a HiDPI
> Gentoo/OpenBox system with no desktop?
> A similar problem exists with Firefox but it can be fixed using a CSS
> file (userChrome.css). However I can't get Thunderbird to work using a
> similar fix. The UI fonts are so small that the app is useless.
> Thanks,
> --
> Valmor

Hi Valmor, I'd think TB also uses CSS to modify the look of the GUI - but I 
don't have it installed on this PC to check its file structure.  This post 
suggests so:

https://support.mozilla.org/en-US/questions/1269224#answer-1282444

I also found this Addon which claims to do just what you're after, but I have 
not tried it out:

https://addons.thunderbird.net/en-US/thunderbird/addon/theme-font-size-changer-for-tb/