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On Custom Markers page is a description on how to create dynamic styles using OpenLayers Styles and variables from View. There is also other approach, using style field in OpenLayers Data Overlay options. This one seems to be much more simple and limited.
There should be some info, which will help to guide decisions abou which approach to use.
Also, I'm not sure if Custom Markers is a correct name for the page, "Dynamic styles" seems more appropriate to me, since we can change not only
externalGraphic, but also other properties.