Providing a simple way for people to view your site in a way that is better for them can be achieved by providing one or more alternative stylesheets.
If you have a lot of written content perhaps a version where the page background is black with text and links in a lighter contrasting colour would be a good option for someone reading late at night, helping your user read next to a sleepy partner with less glare (and later to get a get a better night's sleep by reducing the amount of blue light from the screen).
Older people might welcome larger text, people with dyslexia might appreciate a cream, yellow or pale green background.
Max's CSS page shows a variety of approaches to allowing your user to customise viewing your site.
http://www.thesitewizard.com/css/switch-alternate-css-styles.shtml
dedicated web creation software (e.g. Dreamweaver)
additional editing software (e.g.Fireworks, PhotoShop, Serif Draw Plus, Serif PhotoPlus, Paint, Audacity, MovieMaker, MoviePlus, GIMP)
other (Internet, Web Server)