App.axaml

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

    <Application.Styles>
        <Style Selector="Button:disabled">
          <Setter Property="Template">
            <ControlTemplate>

              <ContentControl Content="{TemplateBinding Content}"
                              ContentTemplate="{TemplateBinding ContentTemplate}"
                              Padding="{TemplateBinding Padding}"
                              HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                              VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                              ToolTip.Tip="Only enabled when a file is selected!"
                              IsEnabled="False"
                              behaviors:ShowDisabledTooltipExtension.ShowOnDisabled="True"
                              Foreground="Gray"/>
            </ControlTemplate>
          </Setter>
        </Style>
        <FluentTheme>
            <FluentTheme.Palettes>
                <ColorPaletteResources x:Key="Light" Accent="#ff00579c" AltHigh="White" AltLow="White"
                                       AltMedium="White" AltMediumHigh="White" AltMediumLow="White"
                                       BaseHigh="Black" BaseLow="#ffcccccc" BaseMedium="#ff898989"
                                       BaseMediumHigh="#ff5d5d5d" BaseMediumLow="#ff737373"
                                       ChromeAltLow="#ff5d5d5d" ChromeBlackHigh="Black"
                                       ChromeBlackLow="#ffcccccc" ChromeBlackMedium="#ff5d5d5d"
                                       ChromeBlackMediumLow="#ff898989" ChromeDisabledHigh="#ffcccccc"
                                       ChromeDisabledLow="#ff898989" ChromeGray="#ff737373"
                                       ChromeHigh="#ffcccccc" ChromeLow="#ffececec" ChromeMedium="#ffe6e6e6"
                                       ChromeMediumLow="#ffececec" ChromeWhite="White" ListLow="#ffe6e6e6"
                                       ListMedium="#ffcccccc" RegionColor="White" />
                <ColorPaletteResources x:Key="Dark" Accent="#ff00579c" AltHigh="Black" AltLow="Black"
                                       AltMedium="Black" AltMediumHigh="Black" AltMediumLow="Black"
                                       BaseHigh="White" BaseLow="#ff333333" BaseMedium="#ff9a9a9a"
                                       BaseMediumHigh="#ffb4b4b4" BaseMediumLow="#ff676767"
                                       ChromeAltLow="#ffb4b4b4" ChromeBlackHigh="Black"
                                       ChromeBlackLow="#ffb4b4b4" ChromeBlackMedium="Black"
                                       ChromeBlackMediumLow="Black" ChromeDisabledHigh="#ff333333"
                                       ChromeDisabledLow="#ff9a9a9a" ChromeGray="Gray" ChromeHigh="Gray"
                                       ChromeLow="#ff151515" ChromeMedium="#ff1d1d1d" ChromeMediumLow="#ff2c2c2c"
                                       ChromeWhite="White" ListLow="#ff1d1d1d" ListMedium="#ff333333"
                                       RegionColor="Black" />
            </FluentTheme.Palettes>
        </FluentTheme>
    </Application.Styles>
</Application>