It is said that websites with a black background are more sustainable from an electricity consumption perspective. Is there a tool to measure the most sustainable website design you can use while developing websites?
Nowadays, a black or white background of websites doesn't matter much as a lot of people trashed their CRT screens already. LCD displays don't vary that much in energy consumption of dark or light colours.
Anyway, to answer your question: No, or at least: difficult.
Why is that? Because today not the computer receiving the website is the huge power consumer but the computer which accepts and processes the requests of webbrowsers.
The best example is google. Current power consumption of Google server parks is estimated as high as one of a city with 500,000 inhabitants. A single search request consumes as much power as required to heat 200ml water.
In short: to measure website's power consumption you should consider the power used to generate the site rather than the power needed to display it.