fix(primitives): use CSS variable for demo component background color

[moonhee0507]

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This pull request:

• Fixes a bug
• Adds additional features/functionality
• Updates documentation or example code
• Other

Summary

Demo components (ScrollArea, Tooltip) had hardcoded background-color: white which doesn't adapt to dark mode. This PR replaces them with var(--color-background) CSS variable to support both light and dark themes.

Related to #851

Screenshots

Before (Dark mode)	After (Dark mode)

Changes

• components/demos/ScrollArea/css/styles.css - Updated background-color to use CSS variable
• components/demos/ScrollArea/css-modules/styles.module.css - Updated background-color to use CSS variable
• components/demos/Tooltip/css/styles.css - Updated background-color to use CSS variable
• components/demos/Tooltip/css-modules/styles.module.css - Updated background-color to use CSS variable

Test URLs

• ScrollArea: http://localhost:3000/primitives/docs/components/scroll-area
• Tooltip: http://localhost:3000/primitives/docs/components/tooltip

(Test in dark mode to verify background colors adapt to the theme)

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