Check checksums of themes

[patrickrobrecht]

Verify that the theme files in the WordPress match the ones in the official WP.org directory to detect compromised files.

There does not seem to be an official as for plugins, but wpcentral.io offers an API endpoint for that. Sample file: https://wpcentral.io/api/checksums/theme/twentytwentyone/1.0

Original idea by @swissspidy in pluginkollektiv/checksum-verifier#1