The same colors are available as block color palettes in the editor’s block settings.

Additionally, Twenty Twenty-One supports several gradient presets for blocks supporting this feature. The image below shows the default gradients in the Columns block settings.

From an accessibility point of view, Dark Mode support is an absolute novelty for a default theme.

Following a discussion on the #core-themes Slack channel, Mel Choyce-Dwan introduced Dark Mode support for Twenty Twenty-One.

At first, it was not clear how this feature could have been implemented in Twenty Twenty-One if added as a theme feature or as a separate project shipping in a plugin.