I use a bookmarklet for that purpose – Zap Colors from https://www.squarefree.com/bookmarklets/zap.html. It resets colors to the browser default while keeping the page layout the same.

Another (which I originally found because of a comment here a few years ago) is http://viewtext.org/.

I like this one because I can add it as a custom search engine in Opera without adding a browser extension or leaving the page.

opera has user style sheets built in. see the "author" and "user" mode. also check out the accessibility layout.

no need to rely on a third party or even tell anyone what you are reading.

I love this app: http://readable.tastefulwords.com/

I've setup a bookmarklet using it; greatest thing since sliced bread.