Sorry, but changing icon sets is only possible if you
- either have got theming support via a theme app (e.g. on CyanogenMod). This is the “low-level” way: you are basically hacking your own smartphone (the SystemUI) to think, that those icons would be the real icons (just like some malware…).
LineageOS does not yet have theming support, but will probably get it (https://forum.xda-developers.com/lineage/help/theme-support-lineage-os-t3531658 ). Standard Android / Fairphone OS probably won’t get it (because those want to be as minimalistic as possible, and icon sets are just ‘extra’ stuff). - or use a Launcher App, that supports custom icon sets. This is the “high-level-app” way. Trebuchet (as of version 6.0.1 on my smartphone) does not support icon sets from within the app. On F-Droid, KISS Launcher ( https://f-droid.org/repository/?fdid=fr.neamar.kiss ) and Open Launcher ( https://f-droid.org/repository/browse/?fdid=com.benny.openlauncher ),
support custom icon sets.
OpenLauncher is developed by the same person
(dkanada, see https://github.com/dkanada) as ICEcons ( https://github.com/dkanada/ICEcons ). If you need a more stable / easy / functional Launcher App (e.g. Apex, Nova), that supports custom icon sets, you need to get one from Google Play (e.g. via Yalp)(unfortunately those are all proprietary).