Update the default themes for 1.16 (#13743)

More or less, as in #13554 

* Dark theme by default (was `system`)
  * Justification:
    > I think the interesting thing that we have today is that the color
    > scheme is dark by default, but our window theme is set to system.
    > System theme in Windows 11 is defaulted to light unless changed by
    > the user. Now, we have a conflict between the theme and color scheme
    > in Terminal.
    > 
    > I think our options become, make the color scheme match "default"
    > and set it to a light color scheme if the system theme is light, or
    > manually match the theme to the color scheme by setting it to dark.
    > 
    > Given that Terminal has historically had a black background with
    > its Campbell color scheme, for a consistent UI experience, I'm
    > voting to change the window theme to be dark by default as well.
* `tab.background: terminalBackground` 
* Change the tab row colors back to "what they were pre-Controlsv2" more
  or less
  - The ramp is based off of "Light tab row background from Edge's
    unfocused titlebar color, inactive tab row color BG from the
    controlsv1 colors"
* `tabRow.unfocusedBackground` set `Transparent`

Closes #13554

src/cascadia/TerminalApp/App.xaml

@@ -149,7 +149,7 @@
                                 See GH #12356 for more history on the subject.
                             -->
                             <SolidColorBrush x:Key="TabViewBackground"
-                                             Color="#0a0a0a" />
+                                             Color="#2e2e2e" />
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
                                             ResourceKey="ApplicationPageBackgroundThemeBrush" />
@@ -165,7 +165,7 @@
                                 GH #12398 has more history on this value, as well as GH #12400
                             -->
                             <SolidColorBrush x:Key="TabViewBackground"
-                                             Color="#dadada" />
+                                             Color="#e8e8e8" />
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
                                             ResourceKey="ApplicationPageBackgroundThemeBrush" />

src/cascadia/TerminalSettingsModel/defaults.json

@@ -26,7 +26,7 @@
     // Miscellaneous
     "confirmCloseAllTabs": true,
     "startOnUserLogin": false,
-    "theme": "system",
+    "theme": "dark",
     "snapToGridOnResize": true,
     "disableAnimations": false,
     "startupActions": "",
@@ -283,20 +283,38 @@
     "themes": [
         {
             "name": "light",
-            "window":{
+            "window": {
                 "applicationTheme": "light"
+            },
+            "tab": {
+                "background": "terminalBackground",
+                "unfocusedBackground": "#00000000"
+            },
+            "tabRow": {
+                "unfocusedBackground": "#FFFFFFFF"
             }
         },
         {
             "name": "dark",
-            "window":{
+            "window": {
                 "applicationTheme": "dark"
+            },
+            "tab": {
+                "background": "terminalBackground",
+                "unfocusedBackground": "#00000000"
+            },
+            "tabRow": {
+                "unfocusedBackground": "#333333FF"
             }
         },
         {
             "name": "system",
-            "window":{
+            "window": {
                 "applicationTheme": "system"
+            },
+            "tab": {
+                "background": "terminalBackground",
+                "unfocusedBackground": "#00000000"
             }
         }
     ],