css rules are very simple to over-write.
first is source comes first, last in source comes last...
meaning the last definition is what you end up with.

Example:

default.css (on every page)
ul{ list-style:none; margin:10px 5px; }

tweaks.css (changes css slightly for specific page)
ul{ list-style: upper-alpha; }

what the browser ends up with is...
"ul{ list-style: upper-alpha; margin:10px 5px; }"
... without actually having to code it all together
Hope that helps...