为什么按钮 没有背景和边框
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="<%=request.getContextPath() %>/js/bootstrap/css/bootstrap.min.css" type="text/css">
<link rel="stylesheet" href="<%=request.getContextPath() %>/js/bootstrap/css/bootstrap-theme.min.css" type="text/css">
<script type="text/javascript" src="<%=request.getContextPath() %>/js/bootstrap/js/jquery.min.js"></script>
<script type="text/javascript" src="<%=request.getContextPath() %>/js/bootstrap/js/bootstrap.min.js"></script>
</head>
<body>
<button type="submit" class="btn">Submit</button>
</body>
</html>
截图请看附件
btn类里没有写背景，加个btn-default就有了