static/style.css

html,
body {
    height: 100%;
    margin: 0;
    padding: 0;
    overflow: hidden;
}

body {
    display: flex;
    flex-direction: column;
}

#canvas-container {
    position: relative;
    flex-grow: 1;
    overflow: hidden;
}

#background-canvas,
#drawing-canvas {
    position: absolute;
    top: 0;
    left: 0;
}

#toolbar {
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 10px;
    background-color: #f0f0f0;
}

#toolbar > * {
    margin: 0 10px;
}