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WordPress provides a mce_css filter, that can be used to add custom stylesheets to the Visual Editor. According to the Codex:

The file can be a .php file, allowing dynamic generation of CSS rules for the content editor.

Example Codex filter callback, modified for a Theme:

function wpse120831_mce_css( $mce_css ) {
    if ( ! empty( $mce_css ) )
        $mce_css .= ',';

    $mce_css .= get_template_directory_uri() . '/dynamic-css.php';

    return $mce_css;
}

add_filter( 'mce_css', 'wpse120831_mce_css' );