editor.css

div#ks-editor #sidepanel {
    resize: horizontal;
    overflow: auto;
    min-width: 250px;
    max-width: 40%;
    height: inherit;
    margin: 0.3rem;
    padding: 0.25rem;

    background-color: white;
    border: 1px lightgrey solid;
    border-radius: 5px;

    overflow-y: auto;
}

div#ks-editor #sidepanel > * {
    margin: 0.25rem;
}

div#ks-editor #content {
    display: flex;
}