It has been solved by modifying the style like
<Style Selector="DataGridCell.right-align > TextBlock">
<Setter Property="TextAlignment" Value="Right" />
<Setter Property="Padding" Value="0,0,4,0" />
</Style>
I've tried this before but the class name wasn't at the right place
<Style Selector="DataGridCell > TextBlock.right-align ">
<Setter Property="TextAlignment" Value="Right" />
<Setter Property="Padding" Value="0,0,4,0" />
</Style>