Add a background to PathIcon.

src/Avalonia.Themes.Default/PathIcon.xaml

@@ -2,16 +2,19 @@
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Style Selector="PathIcon">
     <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundColor}" />
+    <Setter Property="Background" Value="Transparent" />
     <Setter Property="Height" Value="{DynamicResource IconElementThemeHeight}" />
     <Setter Property="Width" Value="{DynamicResource IconElementThemeWidth}" />
     <Setter Property="Template">
       <ControlTemplate>
-        <Viewbox Height="{TemplateBinding Height}"
-                 Width="{TemplateBinding Width}">
-          <Path Fill="{TemplateBinding Foreground}"
-                Data="{TemplateBinding Data}"
-                Stretch="Uniform" />
-        </Viewbox>
+      <Border Background="{TemplateBinding Background}">
+          <Viewbox Height="{TemplateBinding Height}"
+                   Width="{TemplateBinding Width}">
+            <Path Fill="{TemplateBinding Foreground}"
+                  Data="{TemplateBinding Data}"
+                  Stretch="Uniform" />
+          </Viewbox>
+        </Border>
       </ControlTemplate>
     </Setter>
   </Style>

src/Avalonia.Themes.Fluent/PathIcon.xaml

@@ -10,16 +10,19 @@
   </Design.PreviewWith>
   <Style Selector="PathIcon">
     <Setter Property="Foreground" Value="{DynamicResource TextControlForeground}" />
+    <Setter Property="Background" Value="Transparent" />
     <Setter Property="Height" Value="{DynamicResource IconElementThemeHeight}" />
     <Setter Property="Width" Value="{DynamicResource IconElementThemeWidth}" />
     <Setter Property="Template">
       <ControlTemplate>
-        <Viewbox Height="{TemplateBinding Height}"
-                 Width="{TemplateBinding Width}">
-          <Path Fill="{TemplateBinding Foreground}"
-                Data="{TemplateBinding Data}"
-                Stretch="Uniform" />
-        </Viewbox>
+        <Border Background="{TemplateBinding Background}">
+          <Viewbox Height="{TemplateBinding Height}"
+                   Width="{TemplateBinding Width}">
+            <Path Fill="{TemplateBinding Foreground}"
+                  Data="{TemplateBinding Data}"
+                  Stretch="Uniform" />
+          </Viewbox>
+        </Border>
       </ControlTemplate>
     </Setter>
   </Style>