plugin provides a series of images generated on https://mapicons.mapsmarker.com/ as standard:
To override the default icons and adapt them to the colors of your graphic charter, you need to upload your own icons to the skins/[NOM_SKIN]/resources/img/map/poi directory of your graphic charter.
The file name must correspond to the POI's name, among those proposed in Ametys: academy.png, administration.png, ...
You can generate your own customized markers on the https://mapicons.mapsmarker.com/
The site offers several styles of icons and lets you customize the color.