[docs-infra] Add color-scheme to document in iframe demos

[KenanYusuf]

Problem

Originally reported here.

When dark mode is enabled and we have scrollable demos displayed in an iframe, users see the following:

Solution

Adding the color-scheme to the document element:

---

Can be tested locally by making the following changes:

diff --git a/docs/data/material/components/buttons/BasicButtons.tsx b/docs/data/material/components/buttons/BasicButtons.tsx
index 60eb1dfe1e..253383b5b6 100644
--- a/docs/data/material/components/buttons/BasicButtons.tsx
+++ b/docs/data/material/components/buttons/BasicButtons.tsx
@@ -4,7 +4,13 @@ import Button from '@mui/material/Button';
 
 export default function BasicButtons() {
   return (
-    <Stack spacing={2} direction="row">
+    <Stack spacing={2} direction="column" sx={{ overflow: 'auto' }}>
+      <Button variant="text">Text</Button>
+      <Button variant="contained">Contained</Button>
+      <Button variant="outlined">Outlined</Button>
+      <Button variant="text">Text</Button>
+      <Button variant="contained">Contained</Button>
+      <Button variant="outlined">Outlined</Button>
       <Button variant="text">Text</Button>
       <Button variant="contained">Contained</Button>
       <Button variant="outlined">Outlined</Button>
diff --git a/docs/data/material/components/buttons/buttons.md b/docs/data/material/components/buttons/buttons.md
index 0b1b19fe71..2303128192 100644
--- a/docs/data/material/components/buttons/buttons.md
+++ b/docs/data/material/components/buttons/buttons.md
@@ -26,7 +26,7 @@ Buttons communicate actions that users can take. They are typically placed throu
 
 The `Button` comes with three variants: text (default), contained, and outlined.
 
-{{"demo": "BasicButtons.js"}}
+{{"demo": "BasicButtons.js","iframe": true}}
 
 ### Text button

[mui-bot]

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[aarongarciah]

Nice!