Set the design system color

[bjohansebas]

It would be great to start using CSS variables to maintain consistency in colors. I've been trying to do this in previous PRs, but I think it's time to define it completely. This way, we could simplify dark mode and solve several issues with !important.

Inspired by how shadcn/ui handles colors, I'm going to leave a list of colors we could use:

Light mode:

--bg: #ffffff (background)
--fg: #383838 (general texts)
--card-bg: #f0f1f3 (header, code blocks, announcement, etc)
--card-fg: #383838
--hover-bg: #484848 (hover background)
--hover-fg: #fafafa (hover foreground)
--border: #d9e1e4 (borders)
--accent: #0e78ce (Links)

Dark mode:

--bg: #0c0c0c
--fg: #fafafa (general texts)
--card: #181818 (header, code blocks, announcement, etc)
--card-foreground: #71717A
--hover-bg: #484848 (hover background)
--hover-fg: #fafafa (hover foreground)
--border: #d9e1e4
--accent: #2b8fe0 (Links, hover background)

cc/: @expressjs/docs-wg