[Feature/1149] Dark mode (#1281)

* Add Windows version helper

* Initial theme support

* Support custom themes

* Update docs

* Honour HighContrast theme. Remove import "C". Refactor

* Small refactor

* Support inactive theme

* Update Docs

website/docs/reference/options.mdx

@@ -47,6 +47,15 @@ func main() {
             DisableWindowIcon:                 false,
             DisableFramelessWindowDecorations: false,
             WebviewUserDataPath:               "",
+            Theme:                             windows.SystemDefault,
+            CustomTheme: &windows.ThemeSettings{
+                DarkModeTitleBar:   windows.RGB(20, 20, 20),
+                DarkModeTitleText:  windows.RGB(200, 200, 200),
+                DarkModeBorder:     windows.RGB(20, 0, 20),
+                LightModeTitleBar:  windows.RGB(200, 200, 200),
+                LightModeTitleText: windows.RGB(20, 20, 20),
+                LightModeBorder:    windows.RGB(200, 200, 200),
+            },
         },
         Mac: &mac.Options{
             TitleBar: &mac.TitleBar{
@@ -375,6 +384,78 @@ Type: string
 
 This defines the path where the WebView2 stores the user data. If empty `%APPDATA%\[BinaryName.exe]` will be used.
 
+### Theme
+
+Name: Theme
+
+Type: `windows.Theme`
+
+Minimum Windows Version: Windows 10 2004/20H1
+
+This defines the theme that the application should use:
+
+| Value           | Description |
+| --------------- | ----------- |
+| SystemDefault   | *Default*. The theme will be based on the system default. If the user changes their theme, the application will update to use the new setting |
+| Dark            | The application will use a dark theme exclusively |
+| Light           | The application will use a light theme exclusively |
+
+
+### CustomTheme
+
+Name: CustomTheme
+
+Type: `windows.CustomTheme`
+
+Minimum Windows Version: Windows 10/11 2009/21H2 Build 22000
+
+Allows you to specify custom colours for TitleBar, TitleText and Border for both light and dark mode, as well as
+when the window is active or inactive.
+
+#### CustomTheme
+
+The CustomTheme struct uses `int32` to specify the colour values. These are in the standard(!) Windows format of:
+`0x00BBGGAA`. A helper function is provided to do RGB conversions into this format: `windows.RGB(r,g,b uint8)`.
+
+NOTE: Any value not provided will default to black.
+
+```go
+type ThemeSettings struct {
+	DarkModeTitleBar           int32
+	DarkModeTitleBarInactive   int32
+	DarkModeTitleText          int32
+	DarkModeTitleTextInactive  int32
+	DarkModeBorder             int32
+	DarkModeBorderInactive     int32
+	LightModeTitleBar          int32
+	LightModeTitleBarInactive  int32
+	LightModeTitleText         int32
+	LightModeTitleTextInactive int32
+	LightModeBorder            int32
+	LightModeBorderInactive    int32
+}
+```
+
+Example:
+```go
+    CustomTheme: &windows.ThemeSettings{
+        // Theme to use when window is active
+        DarkModeTitleBar:   windows.RGB(255, 0, 0),   // Red
+        DarkModeTitleText:  windows.RGB(0, 255, 0),   // Green
+        DarkModeBorder:     windows.RGB(0, 0, 255),   // Blue
+        LightModeTitleBar:  windows.RGB(200, 200, 200),
+        LightModeTitleText: windows.RGB(20, 20, 20),
+        LightModeBorder:    windows.RGB(200, 200, 200),
+        // Theme to use when window is inactive
+        DarkModeTitleBarInactive:   windows.RGB(128, 0, 0),
+        DarkModeTitleTextInactive:  windows.RGB(0, 128, 0),
+        DarkModeBorderInactive:     windows.RGB(0, 0, 128),
+        LightModeTitleBarInactive:  windows.RGB(100, 100, 100),
+        LightModeTitleTextInactive: windows.RGB(10, 10, 10),
+        LightModeBorderInactive:    windows.RGB(100, 100, 100),
+    },
+```
+
 ## Mac Specific Options
 
 ### TitleBar