Declare.ts

class Person {
    // Extra logic here
}

class AwesomePerson extends Person {
    public doAwesomeStuff(): void {
        
    }
}

class Theme {
    users: Person[] = [];
    // Extra logic here
}

class DarkTheme extends Theme {
    declare users: AwesomePerson[];

    public doStuff() {
        for (var user of this.users) {
            user.doAwesomeStuff();
        }
    }
    
    // Extra logic here
}