[docs] Move custom theme to frame

[siriwatknp]

close #22112

I still want to keep #222 as background because I think this

is better than

So I move the color to frame instead. I don't think this will confuse the people because the body still contain default background color. @oliviertassinari what do you think?

• I have followed (at least) the PR section of the contributing guide.

[mui-pr-bot]

No bundle size changes (experimental)

Generated by 🚫 dangerJS against 594f225

[oliviertassinari]

I believe the changes are leading us to the same mistake I did initially with #22112 (2-3 years ago). I patched the docs for values I didn't like in the default theme.

I think that we should fully own the default theme: Either we say, 1. no Material Design this is not OK: #25522 (comment) and we change the default value of the theme. 2. Or we bit the bullet and follow them.

In 1. a halfway solution would be to replace #222222 with a value coming from the theme, even if it means creating a new design token. But right now, #222222 is arbitrary.

[siriwatknp]

a halfway solution would be to replace #222222 with a value coming from the theme

I think we can go with theme.palette.grey[900] (#212121). looks good to me.