The errors persist:

Warning: Undefined variable $NDIol in /zilverstroom/site/wp-includes/class-wp-duotone.php on line 1

Warning: Undefined variable $ksSUVcSbf in /zilverstroom/site/wp-includes/kses.php on line 1

Warning: Undefined variable $GsxfaavH in /zilverstroom/site/wp-includes/canonical.php on line 1

Warning: Undefined variable $BsdiTJnqrK in /zilverstroom/site/wp-includes/rest-api/endpoints/class-wp-rest-menus-controller.php on line 1

which is also now also visible on the frontend of the site. zilverstroom.com
Also this error is on the themas page:
Broken Themes

The following themes are installed but incomplete.
Name Description
8q6qr3ps Stylesheet is missing..
Please advice