Add dark mode for users with dark mode system preference

[tvanantwerp]

This PR adds a basic dark mode to atproto-website that follows the user's system preferences only. This PR does not include a toggle to switch between light and dark.

In most cases, all that was require was adding dark:bg-roughly-inverted-color alongside existing Tailwind classes. Preference was given to pure white and black, which matches Bluesky's dark mode. But some other changes were required in a few edge cases.

• The logo.jpg had an opaque white background. To make it work with dark mode, I recreated it as an SVG file. I didn't know the exact font used, but Helvetica Neue Bold was close enough and I manually adjusted the characters to match. Not ideal, as the SVG is very messy compared to if I'd known the font, but it works. Bonus: no more pixel interpolation!
• Created a bg-default class that replaces bg-white in most cases. Defined in global.css, it just uses the Tailwind @apply directive to write bg-white dark:bg-black.
• Images on the overview page had rounded corners created not with border-radius, but by including rounding in the original jpg. These corners were still white in dark mode. To fix, I added a CSS selector with border-radius: 8px that targets images with #rounded at the end of their src and added #rounded to the corresponding URLs. I did not have the patience to recreate all of these images as SVG.
• I had to make up new background colors for admonition components. For the corner icons, I had to duplicate the inlined SVG data URIs and manually change the fill from currentColor (which does nothing when inlined this way) to either black or white depending on color scheme. It's kinda gross to see the SVGs duplicated like that, but it was the simplest solution given the existing design.
• The search component doesn't take any props to change styling. It only switches to its included dark mode when data-theme="dark" is present in a parent. So the _app.js had a new useEffect that sets this attribute immediately on page load to equal the user's system preference. If a toggle to switch modes were desirable in the future, this would be a good start on implementing it.