At the begening of the web css didn't exist yet ; so if you wanted to change for eg. the color of your backgrounds, you had to change individually each pages. Fortunately, a few years ago, css was created, and with a single parameter you can change all your website aspect. All ? Not really ! in fact, there's still images, where informations are stored. So I'm thinking about a new system, where pics colors would be defined in the css files too and picture would only be transparent or anything else "neutral"
I may be not the first one to think about it, and by the way, do you know if such system exists ordo you any project related to this ?