We did not change the font. The default font was always Segeo UI. Can you verify that your application window has Segeo UI as font?
What we did change, is not setting TextOptions.TextFormattingModeProperty anymore.
The default value if WPF for that mode is TextFormattingMode.Ideal and till then we overwrote this with TextFormattingMode.Ideal, so in fact there should be no visible change.
If you want to set this mode to Ideal in your application you can do this by simply setting TextOptions.TextFormattingMode="Ideal" on your windows or overwrite the default metadata before any control is created in your application.
But as the mode should already be set to Ideal (because this is the default in WPF) there should be no visible changes. If you can see any changes, it would be very interesting to see some screenshots showing the differences.
As to your concern:
U use the RibbonWindow and as we have to set some properties for this window (FontFamily, FontSize, FontWeigth, Foreground etc.) those property-values get inherited by all other controls which are in the visual tree of the window.