Again, it is not one, it is a few. The icon in release/freedesktop is used for the Linux starter icon from what i remember.
I think the icon in the header is in release\datafiles\blender_icons.svg . And it is not done to simply replace it in the svg iconsheet. You need to run the python script to create the dat files. See linked tutorial. The alert_icons.svg does also have a python script to update the icons. And this Blender icon is used fo the taskbar afaik.
Kind regards
Reiner
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