Something that I've found extremely helpful is "reader view" in Firefox. It doesn't always work, but when it does it's wonderful -- it finds the main text of a web page and shows it in one column with nothing on the side. You can change text size, column width, and color theme -- only a limited set of color themes but they're all highly readable.

On DW, if you have a readable journal style you can use it everywhere. I tend to do that selectively because I like to see what styles other people are using, but when I find someone's bad choices making their journal unreadable I just append ?style=mine to the page's URL.