6.1 / 9.1 Failed to load theme 'https://mydomain.net/ocistheme/ocis.theme.json', invalid theme. Using default theme

Infinite Scale
1

Trying since days to change the colors of ocis.
It looks like my theme gets loaded, but I get this error.

index.html-CANegSco.mjs:5 Failed to load theme 'https://mydomain.net/ocistheme/ocis.theme.json', invalid theme. Using default theme. ZodError
    at get error (PortalTarget.vue_vue_type_script_lang-CzmAS5E0.mjs:22:371590)
    at Qt.parse (PortalTarget.vue_vue_type_script_lang-CzmAS5E0.mjs:22:373694)
    at qT (index.html-CANegSco.mjs:5:179648)
    at async Fa (index.html-CANegSco.mjs:5:191856)
    at async Promise.all (/files/spaces/personal/index 3)
    at async Object.xA [as bootstrapApp] (index.html-CANegSco.mjs:5:201617)

Since this is a .js error, how to debug this.
I tried a minimal ocis.theme.json, a big one, everything… So I think it is not the .json file, that gives the error.
Thx
Chris