WP Customizer compare and set a value via javascript

You’re binding changes to themeb_scheme via an anonymous callback function with the parameter value as the new setting of themeb_scheme.

This should work:

wp.customize( 'themeb_scheme', 'themeb_footerbg', function( themeb_scheme, themeb_footerbg ) {
  themeb_scheme.bind( function( value ) {

    if( value == 'light'){
      themeb_footerbg.set( '#eaeaea' );
    } else if( value == 'dark'){
      themeb_footerbg.set( '#323232' );
    } else  {
      themeb_footerbg.set( '#828282' );
    }
} );