Fix style of input floating window

I use [dressing.nvim](https://github.com/stevearc/dressing.nvim) to improve VIM basic UI elements. I noticed the input style should probably improved. The black background behind the title doesn't feel right.

<img width="304" alt="image" src="https://github.com/spaceduck-theme/nvim/assets/583657/950d6af6-56d4-422d-afc0-c75380e0b187">
