As you can see from this gallery, many designers have built web sites and applications that employ a different color scheme depending on the time of day. It often has a charming effect, but it can probably accomplish something more. Instead of focusing on whether we can do such a thing, I’d like to pose the question of whether we should.
How users might meaningfully benefit
Perhaps the most important reason to change the look of your site between day and night is eye fatigue. At Mix 10, I learned from Kevin Larson, a leading readability expert at Microsoft, that the light emitted from a computer screen does not contribute to eye fatigue if it is in a well-lit room. I presume it follows that a dimmer-looking layout could help reduce eye fatigue in a dimly-lit environment. This argument brings me to a line of thought worth running with.
We must be mindful of the context in which users experience our work. For example, changing a site to light text against a dark background at night would reduce the amount of light emitted by many smartphones, making the use of these devices less irritating to those near a phone user in a dimly-lit restaurant or bar.
Maybe a tablet user likes to read before bed, and reducing the amount of light coming from the screen will help them avoid sending their brain an unwanted signal that it isn’t bed time. Likewise, reading text with a screen blasting the light of a white background in the morning might give a cup of joe an extra kick.
Undeniably, there are some compelling reasons to play with this technique, that bare a lot more weight than “because it would look cute.”
Reasons not to do it
The first and foremost reason I can imagine not to create separate day/night themes is money. Especially in an ad agency, more design work means more design time, and more implementation time, but probably not more budget. Or, perhaps even worse: Clients could make cuts to potentially more important pieces of a project’s scope in order to make room in the budget.
Another potential reason not to change a site’s design with the time of day is because some users may find it annoying. Power users are very choosy about how applications and text look on their screen, and would be rattled if a designer took that decision away.
Make it about the user, not the designer
While I often admire design for design’s sake, we all know it doesn’t work for most projects. Everything we design is a product, which inherently has a market. If the market doesn’t adopt the product we design, it may well be because our design failed. As with any other design technique, the results are a lot more positive if you focus on how to use that technique to please the user, instead of fulfilling an artistic vision.