Parse error: syntax error, unexpected '$theme' (T_VARIABLE) in /......./wp-content/themes/optima/lib/constants.php on line 1

 or perhaps

PHP Parse error: syntax error, unexpected '$theme' (T_VARIABLE) in /..../wp-content/themes/Lightbox/lib/constants.php on line 1

what to do:

Simply re-upload the constants.php file.

why?

When you upload the theme with FTP some of your file such as constans.php are compressed and spaces removed from file. That issue comes from some host. Trey to re-upload files that have errors.

seriously.