I have a Vue SPA which has an OpenLayers 6 map. Said Map is updated in a for loop which waits for rendercomplete events to fire to export the map canvas using the following function :

getMapCanvas () {
    var mapCanvas = document.createElement('canvas');
    var divElement = document.querySelector(".map");
    mapCanvas.width = divElement.offsetWidth;//size[0];
    mapCanvas.height = divElement.offsetHeight;//size[1];
    var mapContext = mapCanvas.getContext('2d');
    Array.prototype.forEach.call(
        document.querySelectorAll('.ol-layer canvas'),
        function (canvas) {
            if (canvas.width > 0) {
                const opacity = canvas.parentNode.style.opacity;
                mapContext.globalAlpha = opacity === '' ? 1 : Number(opacity);
                const transform = canvas.style.transform;
                const matrix = transform
                                        .match(/^matrix\(([^\(]*)\)$/)[1] //eslint-disable-line
                                        .split(',')
                                        .map(Number);
                CanvasRenderingContext2D.prototype.setTransform.apply(mapContext,matrix);
                mapContext.drawImage(canvas, 0, 0);
            }
        }
    );
    return mapCanvas;
},

The problem is that after I turn on dark mode using :

this.map.getLayers().getArray()[0].on(['postrender'], (evt) => {
    if ( this.darkOSM !== null && this.darkOSM === false ) {
        evt.context.globalCompositeOperation = 'color';
        evt.context.fillStyle = 'rgba(0,0,0,' + 1.0 + ')';
        evt.context.fillRect(0, 0, evt.context.canvas.width, evt.context.canvas.height);
        evt.context.globalCompositeOperation = 'overlay';
        evt.context.fillStyle = 'rgb(' + [200,200,200].toString() + ')';
        evt.context.fillRect(0, 0, evt.context.canvas.width, evt.context.canvas.height);
        evt.context.globalCompositeOperation = 'source-over';
        document.querySelector('canvas').style.filter="invert(99%)";
    } 
});

Instead of getting the first image below (dark mode I see on the screen) I get the second image below :