fix centralization of scroll buttons

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

@@ -65,7 +65,7 @@
         <sys:Double x:Key="FontSizeNormal">12</sys:Double>
         <sys:Double x:Key="FontSizeLarge">16</sys:Double>
 
-        <sys:Double x:Key="ScrollBarThickness">20</sys:Double>
-        <sys:Double x:Key="ScrollBarThumbThickness">9</sys:Double>
+        <sys:Double x:Key="ScrollBarThickness">18</sys:Double>
+        <sys:Double x:Key="ScrollBarThumbThickness">8</sys:Double>
     </Style.Resources>
 </Style>

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

@@ -68,7 +68,7 @@
         <sys:Double x:Key="FontSizeNormal">12</sys:Double>
         <sys:Double x:Key="FontSizeLarge">16</sys:Double>
 
-        <sys:Double x:Key="ScrollBarThickness">20</sys:Double>
-        <sys:Double x:Key="ScrollBarThumbThickness">9</sys:Double>
+        <sys:Double x:Key="ScrollBarThickness">18</sys:Double>
+        <sys:Double x:Key="ScrollBarThumbThickness">8</sys:Double>
     </Style.Resources>
 </Style>

src/Avalonia.Themes.Default/ScrollBar.xaml

@@ -10,7 +10,7 @@
                           Grid.Row="0"
                           Focusable="False"
                           MinHeight="{DynamicResource ScrollBarThickness}">
-              <Path Data="M 0 5 L 10 5 L 5 0 Z" />
+              <Path Data="M 0 4 L 8 4 L 4 0 Z" />
             </RepeatButton>
             <Track Grid.Row="1"
                    Grid.Column="1"
@@ -38,7 +38,7 @@
                           Grid.Column="2"
                           Focusable="False"
                           MinHeight="{DynamicResource ScrollBarThickness}">
-              <Path Data="M 0 0 L 5 5 L 10 0 Z" />
+              <Path Data="M 0 0 L 4 4 L 8 0 Z" />
             </RepeatButton>
           </Grid>
         </Border>
@@ -57,7 +57,7 @@
                           Grid.Column="0"
                           Focusable="False"
                           MinWidth="{DynamicResource ScrollBarThickness}">
-              <Path Data="M 5 0 L 5 10 L 0 5 Z" />
+              <Path Data="M 4 0 L 4 8 L 0 4 Z" />
             </RepeatButton>
             <Track Grid.Row="1"
                    Grid.Column="1"
@@ -84,7 +84,7 @@
                           Grid.Column="2"
                           Focusable="False"
                           MinWidth="{DynamicResource ScrollBarThickness}">
-              <Path Data="M 0 0 L 5 5 L 0 10 Z"  />
+              <Path Data="M 0 0 L 4 4 L 0 8 Z"  />
             </RepeatButton>
           </Grid>
         </Border>