[SlartiBartFastFjord]

I'm not sure what you are mean. Do you mean special characters / symbols of a font?

If yes:
It depends on the used font of the skin.
Estuary is using Noto Sans, other skins are using something different.
Since a lot of skins have multiple font support for Asian letters for example, I highly suggest you to use a own font for icons like Font Awesome or https://materialdesignicons.com/ (that's what I'm using)

Thanks, that set me on the right track. I wanted to add another button (open android settings - StartAndroidActivity("", "android.settings.SETTINGS")) to an existing skin (Pellucid). I used Nexus Font to look at the character map of the font used in the skin.

Cheers,
S