To use custom icons, “Theme support” needs to be enabled. Note that this is an experimental feature. It was enabled in Audacity 1.3.8 but not in later versions.
To enable Theme support in the current version of Audacity requires building Audacity from the source code and enabling theme support.
Further information here: https://forum.audacityteam.org/t/waveform-spectrogram-background/24128/2
Audacity 1.3.8 is not recommended for normal use, but may be an easy way to experiment with custom icons and is available here: http://alturl.com/hwyug