How can I apply CSS style changes just for one page?
You don't even need a separate CSS file necessarily. You can add classes to your body for various purposes, identifying page or page type being one of them. So if you had:
<body class="page5">
Then in your CSS you could apply:
.page5 #content {
height: XXXpx;
}
And it would only apply to that page as long as it occurs after your main #content definition.
Just re-define it somewhere after your @import directive:
#content { height: 456px }
for identical CSS selectors, the latter rule overwrites the former.