Hello Tuncay Bayburt !

It seems your current template has the following code (possible in modules\boonex\decorous\data\template\system\css\general.css file):

@media only screen and (min-width: 720px) {

  .cd-nav {

    display: block;

    float: right;

    height: 100%;

  }

}

You need to delete the height: 100%; line and clear the cache in the Studio->Dashboard app.