Fix: The AppFlowy logo at top bar reduces readability in light mode

# Issue

As shown in the video below the black text does not go well with the background color of the top bar. Here are the options I would suggest:

1. Only use the logo with white text across dark and light mode.
2. Change the top bar background color to some light shade.

## Before


## After


The logo now looks readable, and goes well with the background.