blockscout/apps/block_scout_web/assets/css/_mixins.scss

@mixin textfield-placeholder($color: #a3a9b5) {
    &::-webkit-input-placeholder {
        color: $color;
    }
    &::-moz-placeholder {
        color: $color;
    }
    &:-ms-input-placeholder {
        color: $color;
    }
    &:-moz-placeholder {
        color: $color;
    }
}