Installing fonts on android
Cyanogen OS stops at Marshmallow and is deprecated; LineageOS 14.1 (along with CM14.1 before it), being fresh on Nougat, has yet to implement a theme engine, so you need to rely on 3rd-party solutions, which usually uses TTF/OTF format fonts. If you still want to use the fonts in the APKs, extract them from the APK package.