Automatically change website style for mobile device

I'm using the very old version of La.plume CMS. My CMS version doesn't support mobile friendly view. But I', lazy to upgrade my website :).

Here's my trick:
Copy the new mobile.css and put it in my host. Then edit the website header PHP file (top.php)

<link rel="stylesheet" media="only screen and (min-width: 661px)" href="stylesheet.css" type="text/css">
<link rel="stylesheet" media="only screen and (max-width: 660px)" href="mobile.css" type="text/css">

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