Added ScrollBar size resources

src/Avalonia.Themes.Default/Accents/BaseDark.xaml

@@ -52,5 +52,8 @@
         <sys:Double x:Key="FontSizeSmall">10</sys:Double>
         <sys:Double x:Key="FontSizeNormal">12</sys:Double>
         <sys:Double x:Key="FontSizeLarge">16</sys:Double>
+
+        <sys:Double x:Key="ScrollBarWidth">10</sys:Double>
+        <sys:Double x:Key="ScrollBarHeight">10</sys:Double>
     </Style.Resources>
 </Style>

src/Avalonia.Themes.Default/Accents/BaseLight.xaml

@@ -52,5 +52,8 @@
         <sys:Double x:Key="FontSizeSmall">10</sys:Double>
         <sys:Double x:Key="FontSizeNormal">12</sys:Double>
         <sys:Double x:Key="FontSizeLarge">16</sys:Double>
+
+        <sys:Double x:Key="ScrollBarWidth">10</sys:Double>
+        <sys:Double x:Key="ScrollBarHeight">10</sys:Double>
     </Style.Resources>
 </Style>

src/Avalonia.Themes.Default/ScrollBar.xaml

@@ -54,7 +54,7 @@
     </Style>
     <Style Selector="ScrollBar:horizontal">
         <Setter Property="Height"
-                Value="10" />
+                Value="{DynamicResource ScrollBarHeight}" />
         <Setter Property="Template">
             <ControlTemplate>
                 <Border Background="{DynamicResource ThemeControlMidBrush}">
@@ -109,15 +109,15 @@
     </Style>
     <Style Selector="ScrollBar:horizontal /template/ Thumb#thumb">
         <Setter Property="MinWidth"
-                Value="10" />
+                Value="{DynamicResource ScrollBarWidth}" />
     </Style>
     <Style Selector="ScrollBar:vertical">
         <Setter Property="Width"
-                Value="10" />
+                Value="{DynamicResource ScrollBarWidth}" />
     </Style>
     <Style Selector="ScrollBar:vertical /template/ Thumb#thumb">
         <Setter Property="MinHeight"
-                Value="10" />
+                Value="{DynamicResource ScrollBarHeight}" />
     </Style>
     <Style Selector="ScrollBar /template/ RepeatButton.repeat">
         <Setter Property="Padding"