Custom font support would be nice. I can't live without nerd font symbols.

[Un1q32]

Or at least stop allow people to go into /Applications/NewTerm.app and replace the ttf files with any they want. I tried replacing the Cascadia Code ttf with the nf Caskaydia Cove patched nerd font and the font was set back to SF Mono and attempting to select the font from the menu gave a prompt to install the font addon (it was already installed)

[kirb]

Definitely planning this!

In the meantime, I think what you're missing is that you also need to set the PostScript names of the font in Fonts.plist. Likely that just means changing CascadiaCode-Roman and CascadiaCode-Roman_Bold to CaskaydiaCove-Roman and CaskaydiaCove-Roman_Bold (I'm guessing the PostScript name, I could be wrong - if so you can find it using Font Book on a Mac, or probably something similar on Windows/Linux). The app simply loads all .ttf/.otf's found inside NewTerm.app when it launches, it still needs to know which of those it needs to pick when you say you want "Cascadia Code". Since after replacing the ttfs it can no longer find CascadiaCode-Roman, it assumes the font pack isn't installed and displays the install prompt.