Dark mode should be a flag rather than a specific theme

I really like the differentiation we can make by choosing different navigation bar colors. The biggest offender in darkmode isn't that bar, but the white background.

Here is an example where I just reset the navigation bar (and gitlab logo color):

As a plus plus, dark-mode would either be user-defined (as on/off) or automatically toggled by the user OS preferences (this is what twitter.com does today... and it looks like this is just something we can do via media queries: https://stackoverflow.com/questions/50840168/how-to-detect-if-the-os-is-in-dark-mode-in-browsers)