The component UserCardBody.vue is applying the correct style, though the class setting is not deep enough and .cdx-icon may overwrite the custom color back to default.
Let's make sure the styles in UserCardBody.vue are deep enough so it overwrites the default from .cdx-icon
.ext-checkuser-userinfocard-icon {
margin-right: @spacing-25;
color: var(--color-subtle);
}
.ext-checkuser-userinfocard-icon.ext-checkuser-userinfocard-icon-blocks {
color: var(--color-icon-warning);
}