styles.css

/* reseting browser styles */
* {
    box-sizing: border-box;
    font-family: system-ui;
}

body {
    margin: 0;
}

header {
    border-bottom: 1px solid grey;
    height: 15vh;
    padding: 20px 10px;
    width: 100%;
    border-top: 1px solid grey;
    border-bottom: 1px solid grey;
}

header h1,
header h2 {
    margin-right: 10px;
    margin-left: 10px;
}

sidebar header.open {
    height: 225px;
}

sidebar header input {
    margin: 10px 0;
}

#thread-list, 
#message-list {
    list-style: none;
    margin: 0;
    padding: 0;
}

#message-list {
    height: 55vh;
    overflow: scroll;
}

#thread-list li {
    border-bottom: 1px solid grey;
}

#thread-list li h2 {
    padding: 20px 10px;
    margin: 0;
}

#message-list li {
    padding: 10px 20px;
    border-bottom: 1px solid grey;
}

#new-message-form {
    height: 100%;
    display: flex;
    flex-direction: column;
}

#new-message {
    height: 30vh;
    border-top: 1px solid grey;
}

#new-message-form textarea {
    height: 100%;
    margin: 10px;
    border-radius: 5px;
}

#new-message-form button {
    height: 50px;
    margin: 10px;
    border-radius: 5px
}