Fix: custom-css.md document

[konyu]

Change to the correct CSS style
In the original document, the CSS design is not applied

[sweep-ai]

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Reviewed Files

react-docs/custom-css.md

Summary of changes made:

` tag. The pull request corrects the CSS syntax by removing an invalid colon after the `input` selector in the `customCSS` property.

No issues found with the reviewed changes

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