For the time being the following workaround can be used:

    <style>
        .k-chart .k-tooltip, .k-sparkline .k-tooltip, .k-stockchart .k-tooltip {
            background-image: url("//:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAWCAYAAADAQbwGAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADNJREFUeNpi/P//vwMDFQELEP8beQb+HTWQYgP/DHoD/466cAR4edRAyg38P6hLbIAAAwCnWhhVsxvdCAAAAABJRU5ErkJggg==") !important;
        }
    </style>