Avalonia/src/Avalonia.Themes.Fluent/DensityStyles/Compact.xaml

<Styles xmlns="https://github.com/avaloniaui" 
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style Selector="TextBlock" >
        <Setter Property="FontSize" Value="14" />
    </Style>
    <Style>
        <Style.Resources>
            <x:Double x:Key="ControlContentThemeFontSize">14</x:Double>
            <x:Double x:Key="ContentControlFontSize">14</x:Double>
            <x:Double x:Key="TextControlThemeMinHeight">24</x:Double>
            <Thickness x:Key="TextControlThemePadding">2,2,6,1</Thickness>
            <x:Double x:Key="ListViewItemMinHeight">32</x:Double>
            <x:Double x:Key="TreeViewItemMinHeight">24</x:Double>
            <Thickness x:Key="TimePickerHostPadding">0,1,0,2</Thickness>
            <Thickness x:Key="DatePickerHostPadding">0,1,0,2</Thickness>
            <Thickness x:Key="DatePickerHostMonthPadding">9,0,0,1</Thickness>
            <Thickness x:Key="ComboBoxEditableTextPadding">10,0,30,0</Thickness>
            <x:Double x:Key="ComboBoxMinHeight">24</x:Double>
            <Thickness x:Key="ComboBoxPadding">12,1,0,3</Thickness>
            <x:Double x:Key="NavigationViewItemOnLeftMinHeight">32</x:Double>
        </Style.Resources>
    </Style>
</Styles>