Update the Theme schema for 1.17 (#14666)

What it says on the tin. 

I finally figured out the right way to validate schema updates in VsCode, which made this a lot faster. Tossed notes in the wiki for next time I do this. 

Closes #14560

doc/cascadia/profiles.schema.json

@@ -252,7 +252,7 @@
           "default": "Campbell",
           "description": "Name of the scheme to use when the app is using dark theme",
           "type": "string"
-        },
+        }
       }
     },
     "FontConfig": {
@@ -1722,6 +1722,122 @@
         }
       ]
     },
+    "ShowCloseButton": {
+      "enum": [
+        "always",
+        "hover",
+        "never"
+      ],
+      "type": "string"
+    },
+    "ThemeColor": {
+      "description": "A special kind of color for use in themes. Can be an #rrggbb color, #rrggbbaa color, or a special value. 'accent' is evaluated as the user's selected Accent color in the OS, and 'terminalBackground' will be evaluated as the background color of the active terminal pane.",
+      "oneOf": [
+        {
+          "pattern": "^#[A-Fa-f0-9]{3}(?:[A-Fa-f0-9]{3}(?:[A-Fa-f0-9]{2})?)?$",
+          "type": "string",
+          "format": "color",
+          "default": "#000000ff"
+        },
+        {
+          "const": "accent",
+          "type": "string"
+        },
+        {
+          "const": "terminalBackground",
+          "type": "string"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "TabTheme": {
+      "additionalProperties": false,
+      "description": "A set of properties for customizing the appearance of the tabs",
+      "properties": {
+        "background": {
+          "description": "The color of a tab when it is the active tab",
+          "$ref": "#/$defs/ThemeColor"
+        },
+        "unfocusedBackground": {
+          "description": "The color of a tab when it is not the active tab",
+          "$ref": "#/$defs/ThemeColor"
+        },
+        "showCloseButton": {
+          "description": "Controls the visibility of the close button on the tab",
+          "$ref": "#/$defs/ShowCloseButton"
+        }
+      }
+    },
+    "TabRowTheme": {
+      "additionalProperties": false,
+      "description": "A set of properties for customizing the appearance of the tab row",
+      "properties": {
+        "background": {
+          "description": "The color of the tab row when the window is the foreground window.",
+          "$ref": "#/$defs/ThemeColor"
+        },
+        "unfocusedBackground": {
+          "description": "The color of the tab row when the window is inactive",
+          "$ref": "#/$defs/ThemeColor"
+        }
+      }
+    },
+    "WindowTheme": {
+      "additionalProperties": false,
+      "description": "A set of properties for customizing the appearance of the window itself",
+      "properties": {
+        "applicationTheme": {
+          "description": "Which UI theme the Terminal should use for controls",
+          "enum": [ "light", "dark", "system" ],
+          "type": "string"
+        },
+        "useMica": {
+          "description": "True if the Terminal should use a Mica backdrop for the window. This will apply underneath all controls (including the terminal panes and the titlebar)",
+          "type": "boolean",
+          "default": false
+        }
+      }
+    },
+    "Theme": {
+      "additionalProperties": false,
+      "description": "A set of properties for customizing the appearance of the window. This controls things like the titlebar, the tabs, the application theme.",
+      "properties": {
+        "name": {
+          "type": "string",
+          "description": "The name of the theme. This will be displayed in the settings UI.",
+          "not": { 
+            "enum": [ "light", "dark", "system" ]
+          }
+        },
+        "tab": {
+          "$ref": "#/$defs/TabTheme"
+        },
+        "tabRow": {
+          "$ref": "#/$defs/TabRowTheme"
+        },
+        "window": {
+          "$ref": "#/$defs/WindowTheme"
+        }
+      }
+    },
+    "ThemePair": {
+      "additionalProperties": false,
+      "description": "A pair of Theme names, to allow the Terminal to switch theme based on the OS theme",
+      "properties": {
+        "light": {
+          "type": "string",
+          "description": "The name of the theme to use when the OS is in Light theme",
+          "default": "light"
+        },
+        "dark": {
+          "type": "string",
+          "description": "The name of the theme to use when the OS is in Dark theme",
+          "default": "dark"
+        }
+      }
+    },
     "NewTabMenu": {
       "description": "Defines the order and structure of the 'new tab' menu. It can consist of e.g. profiles, folders, and separators.",
       "type": "array",
@@ -2132,14 +2248,23 @@
           "type": "string"
         },
         "theme": {
-          "default": "system",
-          "description": "Sets the theme of the application. The special value \"system\" refers to the active Windows system theme.",
-          "enum": [
-            "light",
-            "dark",
-            "system"
-          ],
-          "type": "string"
+          "default": "dark",
+          "description": "Sets the theme of the application. This value should be the name of one of the themes defined in `themes`. The Terminal also includes the themes `dark`, `light`, and `system`.",
+          "oneOf": [
+            {
+              "type": "string"
+            },
+            {
+              "$ref": "#/$defs/ThemePair"
+            }
+          ]
+        },
+        "themes": {
+          "description": "The list of available themes",
+          "items": {
+            "$ref": "#/$defs/Theme"
+          },
+          "type": "array"
         },
         "showTabsInTitlebar": {
           "default": true,