materio-d8/web/themes/custom/materiotheme/assets/styles/mdi/scss/_icons.scss

@each $key, $value in $mdi-icons {
    .#{$mdi-css-prefix}-#{$key}::before {
        content: char($value);
    }
}

.#{$mdi-css-prefix}-blank::before {
    content: "\F68C";
    visibility: hidden;
}