All colors are defined in generic.xaml
For button, you can see
<StaticResource x:Key="ButtonBackground" ResourceKey="ControlFillColorDefaultBrush" />
<StaticResource x:Key="ButtonBackgroundPointerOver" ResourceKey="SystemColorHighlightTextColorBrush" />
// etc...
then
<SolidColorBrush x:Key="ControlFillColorDefaultBrush" Color="{StaticResource ControlFillColorDefault}" />
then
<Color x:Key="ControlFillColorDefault">#0FFFFFFF</Color>
So they are partially transparent, depending on the parent background color
an can be changed in XAML in ResourceDictionary.ThemeDictionaries for example