implement light mode

there's no implementation for light mode

![light mode](https://github.com/junhoyeo/react-threads/assets/55207584/2b5ebad7-7189-402f-9857-4aac1a9c34b7)
![dark mode](https://github.com/junhoyeo/react-threads/assets/55207584/b199a8d1-1d8a-44b7-b1a1-68693874721b)

possible solutions:

[shadcn + css variables](https://ui.shadcn.com/)

