Are there any free timeline skins/styles available that can be selected instead of the default one ?
Is it possible to add graphical markers or on the time line like a red line that spans the period of some sort of alert?
Are you looking to customize the timeline that appears at the bottom part of the Cesium viewer?
I am currently not aware of any timeline styles that are available as plugins. I will check in with the rest of the Cesium development team. In the meantime, check out these threads.
It seems like other community members have had similar use cases in the past. Let me know if you have any other questions or concerns! I am looking forward to hearing from you.