H!
I tired to add these two Google fonts, but they were not on the list:
https://fonts.google.com/specimen/Zalando+Sans
https://fonts.google.com/specimen/Zalando+Sans+Expanded
I enqueued them by code, because adding them manually in the UI wasn’t working.
I asked Claude Code to investigate. It found this. I have no idea if that is the actual problem, but it might help. 
Found two related bugs while adding custom Google Fonts via the cs_font_data filter (fonts not yet in Cornerstone’s bundled list).
Environment: Pro theme (latest), PHP 8.2, WordPress 6.9.2
Bug 1: PHP warnings when uploading custom fonts via UI
Uploading fonts through Globals > Fonts > Custom Fonts produces repeated PHP warnings:
PHP Warning: Undefined array key "_id" in .../Services/GlobalFonts.php on line 437
locateCustomItem() iterates customFontItems and accesses $value['_id'] directly. The auto-fix at lines 184-186 generates missing _id values, but it doesn’t always execute before locateCustomItem() is called.
Suggested fix:
// Line 437 - add isset() check
if (isset($value['_id']) && $id === $value['_id']) {
Bug 2: cs_font_data filter doesn’t affect the editor font picker
Adding fonts via the documented cs_font_data filter works for frontend resolution but the fonts never appear in the Cornerstone editor font picker.
Root cause: font_data() caches its result in $this->data on line 60. The filter output includes third-party additions, but the cache does not. When font_data_with_load_items() (used by cs_app_font_data for the editor) calls font_data() , it gets the cached array without the filtered additions.
Workaround: Hook into both filters:
add_filter( 'cs_font_data', 'my_custom_fonts' );
add_filter( 'cs_app_font_data', 'my_custom_fonts' );
Suggested fix: Either cache the filtered result instead of the raw merge, or document that developers must hook into both cs_font_data and cs_app_font_data .
Hope these help.