Dark mode fixes: Filter input's textbox is too dark

<img width="365" alt="image" src="https://github.com/HaxeFlixel/flixel-docs/assets/22229331/0cc1b46b-3b8c-4424-b98e-530fc2f41817">
when you type in something, its a bit too dark! 
<img width="342" alt="image" src="https://github.com/HaxeFlixel/flixel-docs/assets/22229331/0835e52f-fe2e-4bc2-b571-1be8116c23bf">
