Styling Avalonia Slider Conntrol

I would like to style the sizes and colors of Avalonia Slider Control.

Visually it seems to work but it stops the sliding functionailty, I tried to look at the Slider source code to find out how to do it properly but cannot figure it out. Here is the code that I am using:

<Border BorderThickness="1" BorderBrush="DarkGray" VerticalAlignment="Center" HorizontalAlignment="Center">
    <Slider Width="200" Height="100" Margin="10" Minimum="120" Maximum="255" Value="210">
        <Slider.Styles>
            <Style Selector="Slider:horizontal">
                <Setter Property="MinWidth" Value="40" />
                <Setter Property="MinHeight" Value="20" />
                <Setter Property="Template">
                    <ControlTemplate>
                        <Grid>
                            <Grid.RowDefinitions>
                                <RowDefinition Height="Auto" />
                                <RowDefinition Height="Auto" MinHeight="20" />
                                <RowDefinition Height="Auto" />
                            </Grid.RowDefinitions>
                            <Border Grid.Row="1" Height="6" Margin="6,0" VerticalAlignment="Center"
                                    Background="Orange" />
                            <Track Name="PART_Track" Grid.Row="1" Orientation="Horizontal">
                                <Thumb Name="thumb" MinWidth="20" MinHeight="20">
                                    <Thumb.Template>
                                        <ControlTemplate>
                                            <Ellipse Width="18" Height="18" StrokeThickness="3"
                                                     Stroke="Crimson" Fill="Green" />
                                        </ControlTemplate>
                                    </Thumb.Template>
                                </Thumb>
                            </Track>
                        </Grid>
                    </ControlTemplate>
                </Setter>
            </Style>
        </Slider.Styles>
    </Slider>
</Border>

It seems to look as expected but it does not slide. Even the initial value is not set.

The problem you are having is you've overridden the template but not included all the template components, so things are now missing. You shouldn't set a control template just to change the Style. Avalonia has a robust "drill down" selector system that allows you to style anything in the template by inheriting the parent selector using the ^. By preceding your Selector with that symbol you can nest styles.

<Style Selector="Slider:horizontal">
    <Setter Property="MinWidth" Value="40" />
    <Setter Property="MinHeight" Value="20" />
    <Setter Property="Background" Value="Orange"/>
    <Setter Property="Foreground" Value="Orange"/>

    <Style Selector="^ Track">
        <Style Selector="^ RepeatButton Border">
            <Setter Property="CornerRadius" Value="0"/>
            <Setter Property="Height" Value="6"/>
        </Style>
                        
        <Style Selector="^ Thumb">
            <Setter Property="Height" Value="18"/>
            <Setter Property="Width" Value="18"/>
            <Setter Property="BorderBrush" Value="Crimson"/>
            <Setter Property="BorderThickness" Value="3"/>
            <Setter Property="Background" Value="Green"/>
        </Style>
    </Style>

</Style>

On the above I did not style the :hover pseudoclass, but I think this should get you the information you need to do it.

The track of the slider is actually a RepeatButton, this lets you click it to make it function. There are two of them, one on either side of the Thumb, so if you want different styles on either side you will have to select them independently by their names (PART_IncreaseButton and PART_DecreaseButton) for example ^ RepeatButton#PART_DecreaseButton Border.