Question

Does anybody know how to change the font of tooltip for ui elements?

Answer 1

This gives you really big tooltips in all places:

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Page.Resources>
      <Style x:Key="{x:Type ToolTip}" TargetType="{x:Type ToolTip}">
         <Setter Property="FontSize" Value="24"/>
      </Style>
   </Page.Resources>
   <Grid>
      <Button Content="Hey" ToolTipService.ToolTip="Hey"/>
   </Grid>
</Page>

If you want to change particular tooltip you can define style closer to that tooltip, or place fontsize directly in the tooltip:

  <Button Content="Hey">
     <ToolTipService.ToolTip>
        <TextBlock FontSize="64" Text="Hey"/>
     </ToolTipService.ToolTip>
  </Button>