Add some initial docs for Themes

TerminalDocs/custom-terminal-gallery/theme-gallery.md

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+---
+title: Windows Terminal Themes Gallery
+description: Some sample theme configurations for Windows Terminal.
+author: zadjii-msft
+ms.author: migrie
+ms.date: 09/08/2022
+ms.topic: sample
+---
+
+# Theme Gallery
+
+Below are some examples of [Themes](/customize-settings/themes.md).
+
+## Seamless
+
+![Windows Terminal Seamless Theme](./../images/seamless-theme-example.gif)
+
+```json
+{
+    "name": "Seamless",
+    "tab":
+    {
+        "background": "terminalBackground",
+        "unfocusedBackground": "#FFFFFF00",
+        "showCloseButton": "always"
+    },
+    "tabRow":
+    {
+        "background": "terminalBackground",
+        "unfocusedBackground": "#2C2C2CFF"
+    },
+    "window":
+    {
+        "applicationTheme": "dark"
+    }
+},
+```
+
+## Under Construction
+
+![Windows Terminal Under Construction Theme](./../images/Under-Construction-Theme.png)
+
+```json
+{
+    "name": "Under Contruction",
+    "tab":
+    {
+        "background": "#FFFF00FF",
+        "showCloseButton": "always",
+        "unfocusedBackground": "#88440088"
+    },
+    "tabRow":
+    {
+        "background": "#FF8800FF",
+        "unfocusedBackground": "#202020FF"
+    },
+    "window":
+    {
+        "applicationTheme": "light",
+        "useMica": true
+    }
+},
+```
+
+(Color scheme is based on [Fun Forrest](https://github.com/mbadolato/iTerm2-Color-Schemes/blob/master/windowsterminal/FunForrest.json))
+

TerminalDocs/customize-settings/themes.md

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+---
+title: Windows Terminal Theme Settings
+description: Learn how to customize the theme settings within Windows Terminal.
+author: zadjii-msft
+ms.author: migrie
+ms.date: 09/06/2022
+ms.topic: how-to
+---
+
+# Theme settings in Windows Terminal
+
+The settings listed below affect the visuals of the Terminal window itself, rather than the appearance of an individual tab/pane. These settings are currently only editable directly in the [settings.json file](../install.md#settings-json-file), and are not configurable via the Settings UI.
+
+```json
+"theme": "dark"
+"themes":
+[
+    // THEME OBJECTS
+]
+```
+
+For some example themes, take a head over to the [Themes Gallery](/custom-terminal-gallery/theme-gallery.md).
+
+Each theme in the `themes` list is comprised of a collection of property objects, that specify the properties of individual elements of the application. For example, the default `"dark"` theme is the following:
+
+```json
+{
+    "name": "dark",
+    "window": {
+        "applicationTheme": "dark"
+    },
+    "tab": {
+        "background": "terminalBackground",
+        "unfocusedBackground": "#00000000"
+    },
+    "tabRow": {
+        "unfocusedBackground": "#333333FF"
+    }
+},
+```
+
+## Theme Name
+
+This is the name of the theme. Names should be unique. The names `dark`, `light`, and `system` are reserved for the built-in default themes.
+
+**Property name:** `name`
+
+**Necessity:** Required
+
+**Accepts:** a string
+
+___
+
+## Window
+
+### Application Theme
+
+This is the name of the color scheme used in the profile. Color schemes are defined in the `schemes` object. More detailed information can be found on the [Color schemes page](./color-schemes.md).
+
+**Property name:** `applicationTheme`
+
+**Necessity:** Optional
+
+**Accepts:** `"system"`, `"dark"`, `"light"`
+
+**Default value:** `"dark"`
+
+___
+
+## Tab Row
+
+These settings are used to configure the appearance of the tab row. When `showTabsInTitlebar` is `true` (the default), this configures the titlebar.
+
+### Background color
+
+The color of the tab row when the window is in the foreground.
+
+**Property name:** `background`
+
+**Necessity:** Optional
+
+**Accepts:** a [Theme Color](#theme-colors).
+
+### Inactive background color
+
+The color of the tab row, when the window is inactive.
+
+**Property name:** `unfocusedBackground`
+
+**Necessity:** Optional
+
+**Accepts:** a [Theme Color](#theme-colors).
+
+___
+
+## Tabs
+
+### Background color
+
+The color a tab should be when the tab is active. Setting a `tabColor` in a profile will override this value. Similarly, setting a color at runtime with the tab color picker will override this color.
+
+This color is always treated as a solid color, even if set to a `terminalBackground` with a pane with an acrylic background.
+
+**Property name:** `background`
+
+**Necessity:** Optional
+
+**Accepts:** a [Theme Color](#theme-colors).
+
+### Inactive background color
+
+The color a tab should be when the tab is inactive. Setting a `tabColor` in a profile will override this value. Similarly, setting a color at runtime with the tab color picker will override this color.
+
+This color is always treated as a solid color, even if set to a `terminalBackground` with a pane with an acrylic background.
+
+When set to `terminalBackground` or `accent`, this will automatically use an alpha value of 30%, to be semi-transparent.
+
+**Property name:** `unfocusedBackground`
+
+**Necessity:** Optional
+
+**Accepts:** a [Theme Color](#theme-colors).
+
+### Show close button
+
+Configures how the "close" button on the tab should behave. This accepts the following values:
+* `"always"`: Always show tab close buttons.
+* `"hover"`: Show the tab close button on the active tab, and any tabs that are hovered with the mouse.
+* `"never"`: Never show tab close buttons. This also disables the ability to close the tab with the middle mouse button.
+
+**Property name:** `showCloseButton`
+
+**Necessity:** Optional
+
+**Accepts:** `"always"`, `"hover"`, `"never"`
+
+**Default value:** `"always"`
+
+___
+
+
+## Theme Colors
+
+The colors used in themes both accept RGBA color values, as well as a few special strings for custom values. The accepted values are as follows
+
+* `"#rgb`, `"#rrggbb`, `"#rrggbbaa`: An RGB color value. When the alpha chanel is omitted, these colors default to a fully opaque alpha channel.
+* `"accent"`: This is a special value that means "the accent color set in the system settings".
+* `"terminalBackground"`: This is a special value evaluated to mean "the background color of the active terminal pane". If there are multiple panes in a tab, then this is the color of the active one. This always uses the `background` of the profile - it ignores anything from a `backgroundImage`, if set.