AvaloniaProject/ApplicationAvalonia/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ApplicationAvalonia.App"
             xmlns:local="using:ApplicationAvalonia"
             RequestedThemeVariant="Dark">
  <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->

  <Application.DataTemplates>
    <local:ViewLocator/>
  </Application.DataTemplates>

  <Application.Styles>

    <FluentTheme>
      <FluentTheme.Palettes>
        <ColorPaletteResources x:Key="Light" Accent="#ffff5500" AltHigh="White" AltLow="White" AltMedium="White" AltMediumHigh="White" AltMediumLow="White" BaseHigh="Black" BaseLow="#bdcccccc" BaseMedium="#e5898989" BaseMediumHigh="#ff5d5d5d" BaseMediumLow="#f2737373" ChromeAltLow="#ff5d5d5d" ChromeBlackHigh="Black" ChromeBlackLow="#bdcccccc" ChromeBlackMedium="#ff5d5d5d" ChromeBlackMediumLow="#e5898989" ChromeDisabledHigh="#bdcccccc" ChromeDisabledLow="#e5898989" ChromeGray="#f2737373" ChromeHigh="#bdcccccc" ChromeLow="#ffececec" ChromeMedium="#f2e6e6e6" ChromeMediumLow="#ffececec" ChromeWhite="White" ListLow="#f2e6e6e6" ListMedium="#bdcccccc" RegionColor="White" />
        <ColorPaletteResources x:Key="Dark" Accent="#ffff5500" AltHigh="Black" AltLow="Black" AltMedium="Black" AltMediumHigh="Black" AltMediumLow="Black" BaseHigh="White" BaseLow="#ff4c4a48" BaseMedium="#ffa6a5a4" BaseMediumHigh="#ffbdbcbb" BaseMediumLow="#ff797876" ChromeAltLow="#ffbdbcbb" ChromeBlackHigh="Black" ChromeBlackLow="#ffbdbcbb" ChromeBlackMedium="Black" ChromeBlackMediumLow="Black" ChromeDisabledHigh="#ff4c4a48" ChromeDisabledLow="#ffa6a5a4" ChromeGray="#ff908e8d" ChromeHigh="#ff908e8d" ChromeLow="#ff21201e" ChromeMedium="#ff2c2a29" ChromeMediumLow="#ff413f3e" ChromeWhite="White" ListLow="#ff2c2a29" ListMedium="#ff4c4a48" RegionColor="#ff141414" />
      </FluentTheme.Palettes>
    </FluentTheme>


    <Style Selector="TextBox">

      <Setter Property="Margin" Value="0 10"/>
      <Setter Property="MaxWidth" Value="200"/>

    </Style>


    <Style Selector="TextBlock.h1">


    </Style>




    <Style Selector="Button">

      <Setter Property="Padding" Value="15 10"/>
      <Setter	Property="Margin" Value="0 10"/>

    </Style>


  </Application.Styles>



</Application>