You could put the ResourceDictionary into the Page.Resources so that all the buttons on that page would use this style. If you want to apply this style to all the buttons in the app, then you could put the ResourceDictionary inside the Application.Resources in the App.xaml file.
Like this:
<Page.Resources>
<ResourceDictionary>
<ResourceDictionary.ThemeDictionaries>
<ResourceDictionary x:Key="Light">
<!--Default colors-->
<SolidColorBrush x:Key="ButtonBackground" Color="Transparent"/>
</ResourceDictionary>
</ResourceDictionary.ThemeDictionaries>
</ResourceDictionary>
</Page.Resources>
Update:
If you want to apply to certain buttons, it would be better to use custom styles.
You could create a default style of Button and change the properties to what you want. You just need to find out which property is using the resources like ButtonBackground and ButtonBackgroundPointerOver in the style. Then you could change the value of these properties to the value you want.
Here is the style that I've changed and you could directly use it. It has the same behavior as the ResourceDictionary.
<Style TargetType="Button" x:Key="TestKey">
<Setter Property="Background" Value="Transparent" />
<Setter Property="BackgroundSizing" Value="OuterBorderEdge" />
<Setter Property="Foreground" Value="DarkSlateBlue" />
<Setter Property="BorderBrush" Value="DarkSlateBlue" />
<Setter Property="BorderThickness" Value="{ThemeResource ButtonBorderThemeThickness}" />
<Setter Property="Padding" Value="{StaticResource ButtonPadding}" />
<Setter Property="HorizontalAlignment" Value="Left" />
<Setter Property="VerticalAlignment" Value="Center" />
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
<Setter Property="FontWeight" Value="Normal" />
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}" />
<Setter Property="UseSystemFocusVisuals" Value="{StaticResource UseSystemFocusVisuals}" />
<Setter Property="FocusVisualMargin" Value="-3" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<ContentPresenter x:Name="ContentPresenter"
Background="{TemplateBinding Background}"
BackgroundSizing="{TemplateBinding BackgroundSizing}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
ContentTransitions="{TemplateBinding ContentTransitions}"
CornerRadius="{TemplateBinding CornerRadius}"
Padding="{TemplateBinding Padding}"
HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
AutomationProperties.AccessibilityView="Raw">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal">
<Storyboard>
<PointerUpThemeAnimation Storyboard.TargetName="ContentPresenter" />
</Storyboard>
</VisualState>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="Transparent" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="MediumSlateBlue" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="MediumSlateBlue" />
</ObjectAnimationUsingKeyFrames>
<PointerUpThemeAnimation Storyboard.TargetName="ContentPresenter" />
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="Transparent" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="MediumSlateBlue" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="MediumSlateBlue" />
</ObjectAnimationUsingKeyFrames>
<PointerDownThemeAnimation Storyboard.TargetName="ContentPresenter" />
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonBackgroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonBorderBrushDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource ButtonForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
</ContentPresenter>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
You could put this style in Page.Resources or Application.Resources, then use it like the following:
<Button Content="Expand Slot" FontWeight="SemiBold" Style="{StaticResource TestKey}" />
You could get more information about custom style here: Create custom styles