Change font color on datagrid

The text color is specified by the rz-cell-data class in the themes. This is why setting it at td level won't work. You can use a column template as in our demo:

            <Template Context="data">
                @if (data.Quantity > 20)
                {
                    <span style='color: green'>@data.Quantity</span>
                }
                else
                {
                    <span style='color: red'>@data.Quantity</span>
                }
            </Template>