You can use a HTML block to define a custom CSS or jQuery to override the original CSS.
jQuery: add a new class and then apply it to your inoput field:
$(document).ready(function() {
$('input[type="text"]').addClass("idleField");
$('input[type="text"]').focus(function() {
$(this).removeClass("idleField").addClass("focusField");
if (this.value == this.defaultValue){
this.value = '';
}
if(this.value != this.defaultValue){
this.select();
}
});
$('input[type="text"]').blur(function() {
$(this).removeClass("focusField").addClass("idleField");
if ($.trim(this.value == '')){
this.value = (this.defaultValue ? this.defaultValue : '');
}
});
});