Там используются HttpOnly cookie. То есть сами cookie есть, но в браузере не видны, также к ним нельзя получить доступ из клиентского кода.
В запросе /check можно увидеть, что в response headers они устанавливаются.

Set-Cookie: tph_uuid=<token>; expires=Wed, 08 Feb 2023 05:42:55 GMT; path=/; secure; HttpOnly

Можно указать конкретное положение цвета процентами

background: linear-gradient(90deg, #333 0%, #f00 20%, #f00 80%, #333 100%);

В этом примере от начала до 20% фона будет переход от #333 к #f00, затем сплошной #f00 от 20% до 80%, от 80% и до конца займет переход с #f00 до #333.

Альтернативный способ сделать такой же градиент:

background: linear-gradient(90deg, #333, #f00, #f00, #f00, #333);

Пофиксил ошибки в коде, теперь работает

const crypto = await import("crypto");

const SHA256 = message => crypto.createHash("sha256").update(message).digest("hex");
const elliptic = await import("elliptic");
const EC = elliptic.default.ec;
const ec = new EC("secp256k1");
const MINT_KEY_PAIR = ec.genKeyPair();
const MINT_PUBLIC_ADDRESS = MINT_KEY_PAIR.getPublic("hex");
const holderKeyPair = ec.genKeyPair();

// const keyPair = ec.genKeyPair();
// public key: key.Pair.getPublic("hex");
// private key: key.Pair.get.Private("hex");

class Block {
    constructor(timestamp = "", data = []) {
        this.timestamp = timestamp;
        this.data = data;
        this.hash = this.getHash();
        this.prevHash = "";
        this.nonce = 0;
    }
    
    getHash() {
        return SHA256(JSON.stringify(this.data) + this.timestamp + this.prevHash + this.nonce);
    }

    mine(difficulty) {
        while(!this.hash.startsWith(Array(difficulty + 1).join("0"))){
            this.nonce++;
            this.hash = this.getHash();
        }
    }

    hasValidTransactions(chain) {
        return this.data.every(transaction => transaction.isValid(transaction, chain));
    }
}

class Blockchain {
    constructor() {
        const initialCoinRelease = new Transaction(MINT_PUBLIC_ADDRESS, holderKeyPair.getPublic("hex"), 100000)
        this.chain = [new Block(Date.now().toString(), [initialCoinRelease])];
        this.difficulty = 1;
        this.blockTime = 30000;
        this.transactions = [];
        this.reward = 297;
    }

    getLastBlock() {
        return this.chain[this.chain.length - 1]
    }

    getBalance(address) {
        let balance = 0;

        this.chain.forEach(block => {
            block.data.forEach(transaction => {
                if(transaction.from === address) {
                    balance -= transaction.amount;
                    balance -= transaction.gas;
                }

                if(transaction.to === address) {
                    balance += transaction.amount;
                }
            })
        })

        return balance;
    }

    addBlock(block) {
        block.prevHash = this.getLastBlock().hash;
        block.hash = block.getHash();

        block.mine(this.difficulty);

        this.chain.push(block);

        this.difficulty += Date.now() - parseInt(this.getLastBlock().timestamp) < this.blockTime ? 1 : -1;
    }

    addTransaction(transaction) {
        if (transaction.isValid(transaction, this)) {
            this.transactions.push(transaction);
        }
        
    }

    mineTransactions(rewardAddress) {
        let gas = 0;

        this.transactions.forEach(transaction => {
            gas += transaction.gas;
        })

        const rewardTransaction = new Transaction(MINT_PUBLIC_ADDRESS, rewardAddress, this.reward + gas);
        rewardTransaction.sign(MINT_KEY_PAIR);

        if (this.transactions.length !==0) this.addBlock(new Block(Date.now().toString(), [rewardTransaction, ...this.transactions]));

        this.transactions = [];
    }

    isValid(blockchain = this) {
        for (let i = 1; i < blockchain.chain.length; i++) {
            const currentBlock = blockchain.chain[i];
            const prevBlock = blockchain.chain[i-1];

            if (
                currentBlock.hash !== currentBlock.getHash() || 
                currentBlock.prevHash !== prevBlock.hash ||
                currentBlock.hasValidTransactions(blockchain)
                ) {
                return false;
            }
        }
        return true;
    }
}

class Transaction {
    constructor(from, to, amount, gas = 0) {
        this.from = from;
        this.to = to;
        this.amount = amount;
        this.gas = gas;
    }

    sign(keyPair) {
        if (keyPair.getPublic("hex") === this.from) {
            this.signature = keyPair.sign(SHA256(this.from + this.to + this.amount + this.gas), "base64").toDER("hex");
        }
    }

    isValid(tx, chain) {
        return (
            tx.from &&
            tx.to &&
            tx.amount &&
            (chain.getBalance(tx.from) >= tx.amount + tx.gas || tx.from === MINT_PUBLIC_ADDRESS && tx.amount === this.reward) &&
            ec.keyFromPublic(tx.from, "hex").verify(SHA256(tx.from + tx.to + tx.amount + tx.gas), tx.signature)
        )
    }
}

const Palihchain = new Blockchain();

// Your original balance is 100000

const girlfriendWallet = ec.genKeyPair();

const transaction = new Transaction(holderKeyPair.getPublic("hex"), girlfriendWallet.getPublic("hex"), 333, 10);

transaction.sign(holderKeyPair);

Palihchain.addTransaction(transaction);

Palihchain.mineTransactions(girlfriendWallet.getPublic("hex"));

console.log("Your balance:", Palihchain.getBalance(holderKeyPair.getPublic("hex")));
console.log("Her balance:", Palihchain.getBalance(girlfriendWallet.getPublic("hex")));

this в обработчике ссылается на класс Client, а не на ServiceFactory. Чтобы исправить можно сделать так
this.#client.on('qr', this.#onQr.bind(this));
либо сделать метод #onQr стрелочной функцией