I was once asked to change the Zabbix icon.
Looking at the code, I found that the following files are used for the icon:
ico file was used in the IE browser, and png images in Google Chrone and others, so I replaced everything, respectively, in the correct format, the pixel sizes of png files themselves can be changed even in Paint, and png to ico converters can be found on the Internet.
After changing the icon, usually over time it will be updated in the browser cache itself, or you can simply clear the browser history.
Also, if the icon files are replaced with the original ones when updating Zabbix, I made a separate backup of them and added items with an interval of 12h and a period for storing history and trends of 1d, an example of a Zabbix agent item key:
As well as triggers that will report that the files have changed, for example:
(last(/Zabbix Server ixnfo.com/vfs.file.cksum[/usr/share/zabbix/favicon.ico],#1)<>last(/Zabbix Server ixnfo.com/vfs.file.cksum[/usr/share/zabbix/favicon.ico],#2))>0