> Putting all 15kb CSS file inline on 100 pages of the site is not smart I think.

Correct, that would be bad. Primarily because the browser won't be able to cache this (or rather it will cache it once for every page!). It will be slower and will use more bandwidth.

I would only consider defining CSS in the HEAD section of the page itself (which I assume is what you mean by "inline") if those CSS classes / IDs are only relevant on that one page and there is no possibility of it being reused. Otherwise the styles should all go in your external CSS file.