Default CSS suggestion

.toggle_title {
    background: #F5F5F5;
    border: 1px solid #DEDEDE;
    line-height: 1em;
    color: #5e6069;
    cursor: pointer;
    padding: 1em;
    margin: 0 0 1.5em 0;
    user-select: none;
    -webkit-user-select: none;
    -moz-user-select: none;
    width: 100%;
}

.toggle_active {
    background: #F5F5F5;
    border: 1px solid #DEDEDE;
    width: 100%;
    margin: 0;
}

.toggle_content {
    background: #F5F5F5;
    color: #515151;
    border-left: 1px solid #DEDEDE;
    border-bottom: 1px solid #DEDEDE;
    border-right: 1px solid #DEDEDE;
    overflow: auto;
    margin: 0 0 1.5em 0;
    padding: 1em;
    width: 100%;
}

https://wordpress.org/plugins/shortcode-toggle/