Making an unfounded assertion that long lines cause eye strain doesn't make your css any better. I certainly don't suffer from such a debilitating limitation. Anyone who does can just make their window narrower, or bump up the font size in their browser.
Also, if you're looking to reduce eye–strain then you shouldn't reduce the contrast of the text with the background color.
Also, if you're looking to reduce eye–strain then you shouldn't reduce the contrast of the text with the background color.