We are looking for the same feature. There is no way to style the focus border when navigating with the keyboard to an individual chart element. Only the highlight color of the entire series.

The function below just hardcodes either black or white based on the chart background via the autoTextColor function.
We use CSS color variables so the parsing actually fails so the focus (stroke) color is always white...

    ChartElement.prototype.toggleFocusHighlight = function toggleFocusHighlight (show) {
        var options = ((this.options || {}).accessibility || {}).highlight || {};
        var focusHighlight = this._focusHighlight;

        if (!show && !focusHighlight) {
            return;
        }

        if (!focusHighlight) {
            var rootBackground = this.getRoot().options.background;
            var highlightColor = autoTextColor(rootBackground);
            var focusHighlightOptions = {
                fill: {
                    opacity: options.opacity,
                    color: options.color
                },
                stroke: Object.assign({}, {color: highlightColor}, options.border),
                zIndex: options.zIndex
            };

            focusHighlight = this._focusHighlight = this.createFocusHighlight(focusHighlightOptions);

            this.appendVisual(focusHighlight);
        }

        focusHighlight.visible(show);
    };