How can I apply CSS style changes just for one page?

Question

I have two css files:

1. A main file (main.css)

2. A specific page file (page5.css). My page.css contains main.css (@import url(main.css));)

Answer 1

Just re-define it somewhere after your @import directive:

#content { height: 456px }

for identical CSS selectors, the latter rule overwrites the former.