My guess would be that you have some sort of branding applied to your tenant.
You can check the branding that is currently applied to your tenant via: https://<tenantDomain>/apps/public/public-options/options.json
This is probably containing the brand-primary variable somewhere and that one is set to #D73237.
In case you don’t want the tenants branding to be applied to your application you can remove the dynamicOptionsUrl in your cumulocity.config.ts file.
The c8y- prefixed variables are used internally, but should not be used by you.