php 切换背景色彩或背景图片或是切换模板是怎样实现的

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<br /><TR class="tr1"><TD width="10%">姓名</TD></TR><br /><TR class="tr1"><TD width="10%">密码</TD></TR><br /><TR class="tr1"><TD width="10%">姓名</TD></TR><br /><TR class="tr1"><TD width="10%">密码</TD></TR><br />