JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" placeholder='Обязуюсь изучать теорию'><button>ОК!</button>

CSS

input {
    padding: 10px;
    border: 3px solid #5bbd9c;
    width: 180px;
    font-size: 12px;
    outline: none;
}

input:focus {
        border: 3px solid #238866;
}

input:focus + button {
    background: #238866;
}

button {
    font-size: 12px;
    padding: 13px;
    border: none;
    background: #5bbd9c;
    box-sizing: border-box;
    color: white;
    outline: none;
    cursor: pointer;
}