style.css

* {
    box-sizing: border-box;
}

body {
    margin: 0px;
    background-color: gray;
}

body.edit {
    background-color: black;
}

#pieces {
    display: none;
}

#controls, #canvas {
    position: absolute;
    left: 0;
    top: 0;
}

#palette {
    position: absolute;
    left: 0;
    top: 50%;
    transform: translate(0, -50%);
}

#buttons {
    position: absolute;
    left: 50%;
    top: 0;
    transform: translate(-50%, 0);
}

#controls, #canvas {
    width: 100vw;
    height: 100vh;
}

#palette, #buttons {
    background-color: rgba(0, 0, 0, .5);
}

body:not(.edit) #palette, body:not(.edit) #controls {
    display: none;
}

#palette use {
    cursor: pointer;
    fill: white;
}

#palette use.disabled {
    fill: #444;
}

#palette use:hover {
    fill: lightgray;
}

#controls .edge {
    fill: rgba(127, 127, 127, .7);
}

#controls .edge.impossible {
    fill: rgba(255, 0, 0, 1);
}

#controls .candidate {
    fill: gray;
}

#buttons > * {
    vertical-align: middle;
}

#buttons > span {
    cursor: pointer;
    color: white;
}

#buttons > .disabled {
    color: gray;
    cursor: default;
}