Hello,
In order to translate the theme settings you will need to use Strings Translation by Polylang.
Read this https://polylang.pro/doc/strings-translation/
The header was designed to be transparent at the top and white when scrolling down. This is the way the theme was designed.
If you want to modify this you will need edit the style.css file with CSS code.
Or you can hire a WordPress developer to help you with this.
