styles.css

@mixin size($w, $h) {
    width: $w;
    height: $h;
}

@mixin bdrs($bdrs) {
    border-radius: $bdrs;
}

%center {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

%clearfix {
    &::after {
        content: "";
        display: block;
        clear: both;
    }
}

@keyframes ball {
    0% { margin-top: 0; }
    20% { margin-top: 3px; }
    80% { margin-top: -5px; }
}

@keyframes pop {
    0% { transform: scale(.6); }
    30% { transform: scale(1.1); }
    60% { transform: scale(.9); }
    100% { transform: scale(1); }
}

body, html {
    margin: 0;
    height: 100%;
    width: 100%;
}

html {
    background: #bdc3c7;
    background: -webkit-linear-gradient(to right bottom, #bdc3c7 , #2c3e50);
    background: linear-gradient(to right bottom, #bdc3c7 , #2c3e50);
}

body {
    font-family: 'Microsoft JhengHei', Arial, sans-serif;
    background: url('https://www.transparenttextures.com/patterns/45-degree-fabric-light.png');
    letter-spacing: 1px;
}

.chat {
    @extend %center;
    @include size(300px, 80vh);
    max-height: 500px;
    @include bdrs(15px);
    background-color: rgba(0, 0, 0, .5);
    overflow: hidden;
}

.chat-title {
    background-color: rgba(0, 0, 0, .3);
    @extend %clearfix;
    @include size(100%, 50px);

    h2 {
        margin: 10px 0;
        padding: 0;
        color: #f0f0f0;
        font-weight: normal;
        font-size: 16px;
        display: inline-block;
        line-height: 30px;
    }

    .avatar {
        float: left;
        margin: 10px;
        padding: 0;
        @include size(30px, 30px);

        img {
            @include bdrs(50%);
            @include size(30px, 30px);
        }
    }
}

.messages {
    @include size(100%, calc(100% - 90px));

    .messages-content {
        @include size(100%, 100%);
        overflow-y: auto;
        box-sizing: border-box;

        .message {
            max-width: calc(100% - 76px);
            padding: 8px;
            margin: 8px 0 8px 12px;
            @include bdrs(5px);
            background-color: rgba(171, 188, 202, .1);
            color: #e3e3e3;
            float: left;
            font-size: 13px;
            position: relative;
            word-break: break-all;
            clear: both;
            animation: pop .3s forwards;

            .timestamp {
                position: absolute;
                right: -40px;
                bottom: 0;
                font-size: 12px;
                color: #bababa;
            }

            &.personal {
                float: right;
                margin: 8px 12px 8px 0;

                .timestamp {
                    right: auto;
                    left: -40px;
                }
            }

            &.typing {
                @include size(30px, 16px);

                &::after {
                    content: "";
                    @include bdrs(50%);
                    @include size(3px, 3px);
                    background-color: white;
                    @extend %center;
                    transform: translate(-50%, -50%);
                    animation: ball 1.2s infinite linear;
                }

                span {
                    @include size(3px, 3px);
                    @extend %center;
                    transform: translate(-50%, -50%);

                    &::before, &::after {
                        content: "";
                        @include bdrs(50%);
                        @include size(3px, 3px);
                        position: absolute;
                        background-color: white;
                        animation: ball 1.2s infinite linear;
                    }

                    &::before {
                        left: 8px;
                        animation-delay: .3s;
                    }

                    &::after {
                        right: 8px;
                        animation-delay: -.3s;
                    }
                }
            }
        }
    }
}

.action-box {
    background-color: rgba(0, 0, 0, .3);
    @include size(100%, 40px);
    @extend %clearfix;

    .action-box-input, .action-box-submit {
        box-sizing: border-box;
        border: 0;
        float: left;
    }

    .action-box-input {
        @include size(calc(100% - 60px), 20px);
        background-color: transparent;
        resize: none;
        color: white;
        margin: 10px 0;
        padding: 0 10px;
        line-height: 20px;
        font-size: 16px;
        letter-spacing: 1px;
        overflow: hidden;

        &::-webkit-input-placeholder {
            line-height: 20px;
            font-size: 13px;
        }
    }

    .action-box-submit {
        @include size(60px, 40px);
        background-color: rgba(189, 195, 199, .2);
        color: white;
        cursor: pointer;
    }
}