Allow switching from dark mode to light mode

[PierreBesson]

Some users like myself prefer light mode to dark mode to the point that it is painful to fill out the survey with the current dark mode. As this survey is supposed to capture the opinion of the entire population of developers and some of those will be bothered by the dark mode, it could even be statistically significant in the results.

Moreover, this could be used to provide interesting statistics on light mode vs dark mode preferences among developers.