If you right click on a Waypoint on the map, or within the GPX Explorer (GE), then you will on the context menu see Styles, and within that menu you can access Waypoint Style. This gives a list of Styles that you have available, simply choose one to apply it to the Waypoint. On the mobiles, simply long press on a Waypoint to reach the Styles menu.
For instance, if you choose ‘Small Icon’, it will apply the Small Icon Style.
To understand the pre-defined Waypoint Styles or to define your own, please open up the Waypoint Style Manager from the Managers Menu on a PC, from Settings->Managers on mobiles, and from the menu on macOS.
Once you have the Waypoint Style Manager open, you can easily see what each of the default Styles has been setup like. If you would like to define your own Styles or edit the default provided Styles, you can do that within the Waypoint Style Manager.
Once you have defined you own Style, the next time you Sync your routes in GPX Manager (GM), your new Style will also be synced, so it will be available on all of your devices.