converse.js/sass/_messages.scss

#conversejs {
    .message {
        .mention {
            font-weight: bold;
        }
        .mention--self {
            font-weight: normal;
        }
        &.date-separator {
            height: 2em;
            margin: 0;
            position: relative;
            text-align: center;
            z-index: 0;

            .separator {
                border-top: 0px;
                border-bottom: var(--chat-separator-border-bottom);
                margin: 0 1em;
                position: relative;
                top: 1em;
                z-index: 5;
            }

            .separator-text {
                background: white;
                bottom: 1px; // Offset needed due to .separator border size
                color: var(--separator-text-color);
                display: inline-block;
                line-height: 2em;
                padding: 0 1em;
                position: relative;
                z-index: 5;
            }
        }

        &.chat-info {
            color: var(--chat-head-color);
            font-size: var(--message-font-size);
            line-height: var(--line-height-small);
            font-size: 90%;
            padding: 0.17rem 1rem;

            &.badge {
                color: var(--chat-head-text-color);
            }
            &.chat-state-notification {
                font-style: italic;
            }
            &.chat-event {
                clear: left;
                font-style: italic;
            }
            &.chat-error {
                color: var(--error-color);
                font-weight: bold;
            }
        }

        .chat-image {
            height: auto;
            width: auto;
            max-height: 15em;
            max-width: 100%;
        }
        &.chat-msg--action {
            font-style: italic;
        }

        &.chat-msg {
            display: inline-flex;
            width: 100%;
            flex-direction: row;
            overflow: auto; // Ensures that content stays inside
            padding: 0.125rem 1rem;

            &.onload {
                animation: colorchange-chatmessage 1s;
                -webkit-animation: colorchange-chatmessage 1s;
            }
            &:hover {
                background-color: rgba(0, 0, 0, 0.035);
                .chat-msg__actions {
                    .chat-msg__action {
                        opacity: 1;
                    }
                }
            }
            &.correcting {
                &.groupchat  {
                    background-color: var(--chatroom-correcting-color);
                }
                &:not(.groupchat) {
                    background-color: var(--chat-correcting-color);
                }
            }

            .spoiler {
                margin-top: 0.5em;
            }
            .spoiler-hint {
                margin-bottom: 0.5em;
            }
            .spoiler-toggle {
                color: white;
                i {
                    color: white;
                    padding-right: 0.5em;
                }
                &:before {
                    padding-right: 0.25em;
                    whitespace: nowrap;
                }
            }

            .chat-msg__content {
                display: flex;
                flex-direction: column;
                justify-content: space-between;
                align-items: stretch;
                margin-left: 0.5rem;
                width: calc(100% - var(--message-avatar-width));
            }
            .chat-msg__content--action {
                margin-left: 0;
            }

            .chat-msg__body {
                display: flex;
                flex-direction: row;
                justify-content: space-between;
                width: 100%;
            }

            .chat-msg__message {
                display: inline-flex;
                flex-direction: column;
                width: 100%;
                overflow-wrap: break-word;
            }

            .chat-msg__edit-modal {
                cursor: pointer;
                padding-right: 0.5em;
            }
            &.headline {
                .chat-msg__body {
                    margin-left: 0;
                }
            }

            .chat-msg__subject {
                font-weight: bold;
                clear: right;
            }

            .chat-msg__text {
                padding: 0;
                color: var(--message-text-color);
                width: 100%;
                white-space: pre-wrap;
                a {
                    word-wrap: break-word;
                    word-break: break-all;
                    display: inline-block;
                }
                img {
                    &.emoji {
                        height: 1.5em;
                        width: 1.5em;
                        margin: 0 .05em 0 .1em;
                        vertical-align: -0.1em;
                    }
                }
                .emojione {
                    margin-bottom: -6px;
                }
            }

            .chat-msg__text--larger {
                font-size: 2em;
                padding-top: 0.25em;
            }

            .chat-msg__media {
                margin-top: 0.25rem;
                word-break: break-all;
                a {
                    word-wrap: break-word;
                }
                audio {
                    width: 100%;
                }
            }

            .chat-msg__actions {
                .chat-msg__action {
                    height: var(--message-font-size);
                    font-size: var(--message-font-size);
                    padding: 0;
                    border: none;
                    opacity: 0;
                    background: transparent;
                    cursor: pointer;
                    &:focus {
                        display: block;
                    }
                }
            }

            .chat-msg__avatar {
                margin-top: 0.5em;
                vertical-align: middle;
                height: var(--message-avatar-height);
                width: var(--message-avatar-width);
                min-height: var(--message-avatar-height);
                min-width: var(--message-avatar-width);
            }

            .chat-msg__heading {
                width: 100%;
                margin-top: 0.5em;
                padding-right: 0.25rem;
                padding-bottom: 0.25rem;
                display: flex;

                .chat-msg__author {
                    overflow: hidden;
                    text-overflow: ellipsis;
                    white-space: nowrap;
                    font-family: var(--heading-font);
                    font-size: 115%;
                    font-weight: bold;
                    padding-bottom: 1px; // Hack to avoid cutting of text on the button
                }
                .badge {
                    margin-left: 0.5em;
                    font-family: var(--normal_font);
                }
                .chat-msg__time {
                    padding-left: 0.25em;
                    padding-right: 0.25em;
                    color: var(--text-color-lighten-15-percent);
                }
            }
            &.chat-msg--action {
                .chat-msg__content {
                    flex-wrap: wrap;
                    flex-direction: row;
                    justify-content: flex-start;
                }
                .chat-msg__text {
                    width: auto;
                }
                .chat-msg__heading {
                    margin-top: 0;
                    padding-bottom: 0;
                    width: auto;

                    .fa {
                        margin-left: 0.5em;
                    }
                }
                .chat-msg__author {
                    font-size: var(--message-font-size);
                }
                .chat-msg__time {
                    margin-left: 0;
                }
            }

            &.chat-msg--followup {
                .chat-msg__heading,
                .chat-msg__avatar {
                    display: none;
                }
                .chat-msg__content {
                    margin-left: 2.75rem;
                }
            }

            .chat-msg__receipt {
                margin-right: 0.5em;
                color: var(--message-receipt-color);
            }
        }
    }

    .chatroom-body .message {
        &.onload {
            animation: colorchange-chatmessage-muc 1s;
            -webkit-animation: colorchange-chatmessage-muc 1s;
        }
        .separator {
            border-top: 0px;
            border-bottom: var(--chatroom-separator-border-bottom);
        }
    }
}

#conversejs.converse-overlayed {
    .message {
        &.chat-msg {
            &.chat-msg--followup {
                .chat-msg__content {
                    margin-left: 0;
                }
            }
        }
    }
}

@media screen and (max-width: 767px) {
    #conversejs:not(.converse-embedded)  {
        .message {
            &.chat-msg {
                .chat-msg__author {
                    white-space: normal;
                }
            }
        }
    }
}