use lato font.

src/Avalonia.Themes.Fluent/Accents/Base.xaml

@@ -12,7 +12,7 @@
     <Color x:Key="SystemColorHotlight">#FF0066CC</Color>
     <Color x:Key="SystemColorWindow">#FFFFFFFF</Color>
     <Color x:Key="SystemColorWindowText">#FF000000</Color>
-    <FontFamily x:Key="ContentControlThemeFontFamily">avares://Avalonia.Themes.Fluent/Assets#Open Sans</FontFamily>
+    <FontFamily x:Key="ContentControlThemeFontFamily">avares://Avalonia.Themes.Fluent/Assets#Lato</FontFamily>
     <sys:Double x:Key="ControlContentThemeFontSize">13</sys:Double>
 
     <SolidColorBrush x:Key="SystemControlTransparentBrush" Color="Transparent" />