JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" class="box_yazi2" id="konu" name="konu" value="Boş" />

JavaScript

$('.box_yazi2').each(function(index, element) {
    var $element = $(element);
    var defaultValue = $element.val();
    $element.css('background-color', '#555555');
    $element.focus(function() {
        var actualValue = $element.val();
        if (actualValue == defaultValue) {
            $element.val('');
            $element.css('background-color', '#3399FF');
        }
    });
    $element.blur(function() {
        var actualValue = $element.val();
        if (!actualValue) {
            $element.val(defaultValue);
            $element.css('background-color', '#555555');
        }
    });
});