We do not have a setting for that and I think doing it in CSS only is difficult, but you can create a theme component to replace it.
In CSS, you will hide the old icon:
.post-notice {
.d-icon-user-shield {
display: none;
}
}
In JavaScript (you put this in <head> of the theme component), you will decorate the notice and add the new icon:
<script type="text/discourse-plugin" version="0.8">
const { iconNode } = require("discourse-common/lib/icon-library");
api.decorateWidget('post-notice:before', helper => {
return iconNode('heart');
});
</script>
… or if you want to replace with an image…:
<script type="text/discourse-plugin" version="0.8">
api.decorateWidget('post-notice:before', helper => {
return helper.h('img', {
src: 'https://www.discourse.org/a/img/home-spot-1.png',
style: 'margin-right: 0.65em',
height: 30,
width: 30
});
});
</script>
With themes and theme components, the sky is the limit. If you want to learn more, we have a resource for that: