How to change the Mail icon for High Sierra

I think this may be something unique to the Applications icons (and specifically mail) as I think normal folders can be changed. I have to say I am surprised that the Mac is not more 'user friendly' in this way.

Applications are protected by System Integrity Protection (SIP). SIP prevents anything from modifying the system files (including installed Applications). The inability to change the icon is a residual effect of that protection.

My mistake, I didn't realize this was due to a discrepancy between High Sierra and Sierra. After looking into it a bit, it appears that this is not possible on High Sierra. You may find this suggestion useful, it mentions trying a more standardized image: https://discussions.apple.com/thread/8104147?answerId=32777258022#32777258022. The most I can recommend is trying an image that adheres to Apple's usual icon size, which appears to range between 16x16 pixels and 512x512 pixels. I hope this is somewhat helpful.

Copy the image you would like to use to your clipboard. Go to File > Get Info for the Mail app. Then, at the top of the Info window, click the picture of the icon, and then choose Edit > Paste. Have not tested this method but should change your image to the image of your choice, assuming you have that image handy. You may also need to restart your Mac for this change to take effect. Let me know if this works out for you.

Last idea I have on this, but you might try hitting delete on on the image the Info pane before pasting in the new one. Found some evidence that this might work here, but again I haven't tested this personally. Let us know if you do find an answer to this problem.

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