Typescript: .d.ts file not recognized

There might be two issues here:

1.) You need to augment Window interface in global scope.

Either remove all import/export keywords to make the containing file a non-module/script. Or you can use declare global in a module to mark everything wrapped in it as global declaration:

import { MyCustomGlobal} from './classes';

declare global {
  interface Window {
    myCustomGlobal: MyCustomGlobal;
  }
}

2.) Equally named files, that only differ in their .ts/.d.ts extension, don’t work. To quote the docs:

Please note that the compiler does not include files that can be possible outputs; e.g. if the input includes index.ts, then index.d.ts and index.js are excluded. In general, having files that differ only in extension next to each other is not recommended.

Make sure to give a different name to index.d.ts (like global.d.ts), so the compiler doesn’t only pick index.ts. global.d.ts will included automatically, if placed somewhere in the TS project marked by tsconfig.json and
if "files" and "include" config options are left unspecified.