docs: use dark theme for devices that prefer dark theme

[aghArdeshir]

What I did

• created a dark theme

Here is a comparison of how this dark theme looks like compared to original theme on mobile

ing-lion-dark-light-theme-compare-mobile.webm

The same comparison for desktop (FLASHING warning)

ing-lion-dark-light-theme-compare-desktop.webm

Here is a video of this settings respecting the OS theme setting (FLASHING warning)

ing-lion-dark-light-theme-respect-os-settings.webm

Alternatives I tried/though of

1. Thought of creating a button that allows switching between dark/light/auto themes, persist in local storage, put it in topbar/sidenav, etc... . But picked delivering a dark theme now over perfecting it.
2. tried moving the dark theme related styles to a separate file and @import url(./dark-theme.css);. But the problem was I had to put the @import at the beginning of the file, and then the dark theme would lose its specifity and gets overriden by white background of body. Chose not to go with !important for this minimal change and keep it simple for now. The benefit of moving to a separate file (later) would be that we can fine-tune all dark-theme-related styles in that file.

This PR fixes #2336

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[aghArdeshir]

Spotted a problem, just above the footer the color changes a bit.

I'll fix that, and in addition will make the dark theme's background not so black! (no more #000000). Hoping it to look like this:

[aghArdeshir]

This PR is also blocked by this: #2339