Implement an initial dark mode

[parlough]

Resolves #3632

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 0766125):

https://flutter-docs-prod--pr11877-feat-dark-mode-toggle-tcfsaj35.web.app

[lamek]

This looks great. I checked about 50 pages and focused on as many UI elements as I could list. The only thing that stuck out was images. Here's one example where the background going dark makes it very hard to see the : https://flutter-docs-prod--pr11877-feat-dark-mode-toggle-tcfsaj35.web.app/ui/layout#conceptual-example

I made a list of .md files that include an image reference: https://gist.github.com/lamek/0d635c930d29f4fbc72e628e93e8e14b

A lot to check. Alternatively, just scanning the images dir for a file count:

86 directories, 879 files
lamenzo@penguin:~/website/src/content/assets/images/docs$

But, I wouldn't consider checking every file a blocker. IMO, spot checking ~50 images, only 1 had an issue.

I think it might be best to tackle this over time, and add some quick guidance when adding an image to preview in both dark/light mode.

[lamek]

"Regular" DartPad has a dark mode. Do you know if it's possible to toggle embedded DartPad to dark mode?

Example page: https://flutter-docs-prod--pr11877-feat-dark-mode-toggle-tcfsaj35.web.app/cookbook/lists/floating-app-bar

[antfitch]

Gorgeous!!!

[sfshaza2]

Thanks for your thorough review, @lamek! We discussed many of these issues in our 1-1-1 meeting today. :) As I stated in the meeting, I feel that this is good enough to land! lgtm