fix: Add menu position top's div button style (fix: #185)

[junghwan-park]

Please check if the PR fulfills these requirements

• It's submitted to right branch according to our branching model
• It's right issue type on title
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Description

• Add div element selector for default UI's menu position top div button

ref: #185

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Thank you for your contribution to TOAST UI product. 🎉 😘 ✨

[jinwoo-kim-nhn]

수고하셨습니다.