[docs] Sync default theme with the theme used by the documentation

[xclw2000]

Take Button as example, your example color is #1976d2, but when show in CodeSandBox, it's color is #3f51b5

[mbrookes]

The documentation uses a custom theme. We've previously discussed aligning them in v5, though I don't recall where exactly. (Possibly another similar issue, of which this would be a duplicate...)

[oliviertassinari]

@mbrookes I couldn't find any specific issue for this too, but we had https://trello.com/c/v7uLsyMB/2601-use-default-theme-in-docs.

[mbrookes]

You mention it in the linked issue #21040 (comment), so we didn't imagine it. 😄

[xclw2000]

I resolved this with a global provider, thank you all

[petrussola]

same here, would be helpful to align docs and reality, I was very confused for a while until I found this issue