Have you tried creating a custom .desktop file in $HOME/.local/share/applications ? You can copy the one from /usr/share/applications/ and change the line about the icon. For the name of the system icons you can use a program like "Icon Explorer".
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there doesn't seem to be anything here
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A different icon theme would be the first stop. The other possibility, if you wish to keep the icon theme. Is to download a PNG or SVG of the icon you want. And then edit the shortcuts icon to point to it.
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