GramJS

const { Api, TelegramClient } = require("telegram");
const { StringSession } = require("telegram/sessions");

const session = new StringSession(""); // You should put your string session here
const client = new TelegramClient(session, apiId, apiHash, {});

(async function run() {
  await client.connect(); // This assumes you have already authenticated with .start()

  const result = await client.invoke(
    new Api.account.InstallTheme({
      dark: true,
      theme: new Api.InputTheme({
        id: BigInt("-4156887774564"),
        accessHash: BigInt("-4156887774564"),
      }),
      format: "some string here",
      baseTheme: new Api.BaseThemeClassic({}),
    })
  );
  console.log(result); // prints the result
})();

import { Api, TelegramClient } from "telegram";
import { StringSession } from "telegram/sessions";

const session = new StringSession(""); // You should put your string session here
const client = new TelegramClient(session, apiId, apiHash, {});

(async function run() {
  await client.connect(); // This assumes you have already authenticated with .start()

  const result: Api.Bool = await client.invoke(
    new Api.account.InstallTheme({
      dark: true,
      theme: new Api.InputTheme({
        id: BigInt("-4156887774564"),
        accessHash: BigInt("-4156887774564"),
      }),
      format: "some string here",
      baseTheme: new Api.BaseThemeClassic({}),
    })
  );
  console.log(result); // prints the result
})();