Introduce default dark theme

[adguernier]

Problem

React-admin offers both a light and dark theme, but since only one theme is enabled by defaumt, the theme switcher doesn't appear in new admins.

Solution

Update the <Admin> component to include dark theme by default. This enables the theme switcher on all apps.

TODO

• Add the defaultDarkTheme in AdminUI
• Remove the default darkTheme in stories
• Update documentation
• Add upgrade guide

[fzaninotto]

Neat!

[fzaninotto]

We have many stories that set a custom dark theme. You should revisit these, too.

[fzaninotto]

Interesting side effect: My OS defaults to dark theme, and so every story in the Storybook that uses <Admin> or <AdminContext> defaults to dark theme, too.

I suggest we systematically set the defaultTheme="light" in stories to avoid that.

[adguernier]

@fzaninotto I revisited stories and specs that used a custom dark theme.