[UI] Improve Theme Switch Button Hover Color

[ATREAY]

Description:

When hovering over the theme switch button in light mode, the color of the light mode icon and the background of the button are the same, resulting in poor visibility of the light mode icon. To enhance the user interface experience, it is recommended to change the color of the light mode icon to black.

You can verify this by visiting the Hyperledger Besu documentation.

Screenshots:

Dark mode:

Light mode:

Do you want to work on this issue?

• [ ] Yes
• [x] No