feat(theme): Fluent foundation — system Light/Dark + brand palette across all windows

Test-SnipIT.ps1

@@ -2904,6 +2904,33 @@ It 'routes Undo and Redo only after editor and draft ownership' {
         -Key Z -Modifiers Ctrl) $null
 }
 
+Describe 'Get-SnipFluentPalette'
+It 'returns the locked light palette' {
+    $p = Get-SnipFluentPalette -Mode Light
+    ShouldBe $p.Accent '#035BA3'
+    ShouldBe $p.OnAccent '#FFFFFF'
+    ShouldBe $p.AccentText '#035BA3'
+    ShouldBe $p.Positive '#043D19'
+    ShouldBe $p.Destructive '#A60707'
+    ShouldBe $p.Neutral '#909496'
+}
+It 'lightens accent text and positive for dark mode contrast' {
+    $p = Get-SnipFluentPalette -Mode Dark
+    ShouldBe $p.Accent '#035BA3'
+    ShouldBe $p.AccentText '#5E9BD0'
+    ShouldBe $p.Positive '#2E6B42'
+    ShouldBe $p.Ground '#000000'
+}
+It 'maps every override key to a palette hex' {
+    $p = Get-SnipFluentPalette -Mode Light
+    foreach ($k in @('AccentFillColorDefaultBrush','AccentButtonBackground',
+            'CheckBoxCheckBackgroundFillChecked','ProgressBarForeground',
+            'TextOnAccentFillColorPrimaryBrush','AccentTextFillColorPrimaryBrush')) {
+        if (-not $p.ResourceMap.ContainsKey($k)) { throw "missing key $k" }
+        if ($p.ResourceMap[$k] -notmatch '^#[0-9A-F]{6}$') { throw "bad hex for $k" }
+    }
+}
+
 Write-Host ""
 $total = $script:Pass + $script:Fail
 $color = if ($script:Fail -eq 0) { 'Green' } else { 'Red' }

scripts/Export-SnipITModules.ps1

@@ -137,7 +137,8 @@ $moduleFunctions = [ordered]@{
         'New-SnipPreviewContext', 'Set-SnipPreviewMenuStyle',
         'Connect-SnipPreviewMenuButton', 'Connect-SnipPreviewTransientContextMenu',
         'New-SnipSplitControl', 'Set-SnipPropertyIsland',
-        'Set-SnipPreviewStatusPresentation', 'Set-SnipPreviewResponsiveMode',
+        'Set-SnipPreviewStatusPresentation', 'Set-SnipPreviewActiveChrome',
+        'Set-SnipPreviewResponsiveMode',
         'Initialize-SnipPreviewAnnotations', 'New-SnipPreviewWindow',
         'Show-PreviewWindow'
     )

src/00-Core.ps1

@@ -1262,6 +1262,74 @@ function Get-SnipThemeTokens {
     }
 }
 
+function Get-SnipFluentPalette {
+    # Pure Fluent brand palette for one theme mode.
+    #
+    # Returns the locked accent/semantic hexes plus ResourceMap: the exact
+    # Fluent resource keys the WPF theme dictionary overrides, each mapped to
+    # one of those hexes. Callers apply ResourceMap verbatim — no WPF types
+    # are touched here so this stays loadable in -CoreOnly on any platform.
+    [CmdletBinding()]
+    param(
+        [Parameter(Mandatory)]
+        [ValidateSet('Light','Dark')]
+        [string]$Mode
+    )
+
+    $isDark = $Mode -eq 'Dark'
+    $accent      = '#035BA3'
+    $accentHover = if ($isDark) { '#0E68B4' } else { '#024B87' }
+    $accentPress = if ($isDark) { '#024B87' } else { '#023E70' }
+    $onAccent    = '#FFFFFF'
+    $accentText  = if ($isDark) { '#5E9BD0' } else { '#035BA3' }
+    $positive    = if ($isDark) { '#2E6B42' } else { '#043D19' }
+
+    $map = @{
+        AccentFillColorDefaultBrush                  = $accent
+        AccentFillColorSecondaryBrush                = $accentHover
+        AccentFillColorTertiaryBrush                 = $accentPress
+        AccentFillColorSelectedTextBackgroundBrush   = $accent
+        TextOnAccentFillColorPrimaryBrush            = $onAccent
+        TextOnAccentFillColorSecondaryBrush          = $onAccent
+        AccentTextFillColorPrimaryBrush              = $accentText
+        AccentTextFillColorSecondaryBrush            = $accentText
+        AccentTextFillColorTertiaryBrush             = $accentText
+        AccentButtonBackground                       = $accent
+        AccentButtonBackgroundPointerOver            = $accentHover
+        AccentButtonBackgroundPressed                = $accentPress
+        AccentButtonForeground                       = $onAccent
+        AccentButtonForegroundPointerOver            = $onAccent
+        AccentButtonForegroundPressed                = $onAccent
+        AccentButtonBorderBrush                      = $accent
+        AccentButtonBorderBrushPointerOver           = $accentHover
+        AccentButtonBorderBrushPressed               = $accentPress
+        CheckBoxCheckBackgroundFillChecked           = $accent
+        CheckBoxCheckBackgroundStrokeChecked         = $accent
+        CheckBoxCheckBackgroundFillCheckedPointerOver = $accentHover
+        CheckBoxCheckBackgroundStrokeCheckedPointerOver = $accentHover
+        CheckBoxCheckBackgroundFillCheckedPressed    = $accentPress
+        CheckBoxCheckBackgroundStrokeCheckedPressed  = $accentPress
+        CheckBoxCheckGlyphForegroundChecked          = $onAccent
+        CheckBoxCheckGlyphForegroundCheckedPointerOver = $onAccent
+        CheckBoxCheckGlyphForegroundCheckedPressed   = $onAccent
+        ProgressBarForeground                        = $positive
+    }
+
+    [pscustomobject][ordered]@{
+        Mode          = $Mode
+        Accent        = $accent
+        AccentHover   = $accentHover
+        AccentPressed = $accentPress
+        OnAccent      = $onAccent
+        AccentText    = $accentText
+        Neutral       = '#909496'
+        Positive      = $positive
+        Destructive   = '#A60707'
+        Ground        = if ($isDark) { '#000000' } else { '#FFFFFF' }
+        ResourceMap   = $map
+    }
+}
+
 function Get-SnipContrastRatio {
     [CmdletBinding()]
     param(

src/10-Bootstrap.ps1

@@ -460,6 +460,89 @@ function Uninstall-SnipIT {
     }
 }
 
+# Reports the Windows app theme as 'Light' or 'Dark'. The registry read is
+# injected through -Reader so tests can pin a value without touching HKCU;
+# a missing or unreadable value falls back to 'Light'.
+function Get-SnipSystemThemeMode {
+    [CmdletBinding()]
+    param(
+        [scriptblock]$Reader = {
+            try {
+                Get-ItemPropertyValue `
+                    -Path 'HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize' `
+                    -Name 'AppsUseLightTheme' -ErrorAction Stop
+            } catch { $null }
+        }
+    )
+
+    $value = & $Reader
+    $numeric = $value -as [int]
+    if ($null -ne $value -and $null -ne $numeric -and $numeric -eq 0) { 'Dark' } else { 'Light' }
+}
+
+# Applies the Fluent palette to one window and returns the mode that was used.
+# Sets the experimental .NET 9 ThemeMode through reflection when the type exists
+# (older runtimes silently keep the resource overrides only), grounds the window
+# background, and writes every ResourceMap entry in as a frozen brush.
+function Initialize-SnipWindowTheme {
+    [CmdletBinding()]
+    param(
+        [Parameter(Mandatory)]
+        [System.Windows.Window]$Window,
+        [ValidateSet('Light','Dark')]
+        [string]$Mode
+    )
+
+    if (-not $Mode) { $Mode = Get-SnipSystemThemeMode }
+    $palette = Get-SnipFluentPalette -Mode $Mode
+
+    # Experimental .NET 9 ThemeMode API — reflection only, absent-type safe.
+    $themeModeType = 'System.Windows.ThemeMode' -as [type]
+    if ($null -ne $themeModeType) {
+        try {
+            $themeMode = [Activator]::CreateInstance($themeModeType, @($Mode))
+            $app = [System.Windows.Application]::Current
+            if ($null -ne $app) {
+                $app.GetType().GetProperty('ThemeMode').SetValue($app, $themeMode)
+            }
+            $Window.GetType().GetProperty('ThemeMode').SetValue($Window, $themeMode)
+        } catch {
+            Write-SnipDiag -Message 'ThemeMode apply failed' -ErrorRecord $_
+        }
+    }
+
+    # Glass surfaces (WindowStyle=None + AllowsTransparency) paint their chrome with
+    # an inner rounded Border and rely on the window itself staying transparent.
+    # Grounding them would write a Local opaque value over the XAML's Transparent and
+    # render a square block behind the rounded corners, so skip grounding entirely.
+    if (-not $Window.AllowsTransparency) {
+        if ($Mode -eq 'Dark') {
+            $ground = [System.Windows.Media.SolidColorBrush]::new(
+                [System.Windows.Media.ColorConverter]::ConvertFromString($palette.Ground))
+            $ground.Freeze()
+            $Window.Background = $ground
+        } else {
+            $Window.SetResourceReference(
+                [System.Windows.Controls.Control]::BackgroundProperty,
+                'ApplicationBackgroundBrush')
+        }
+    }
+
+    foreach ($entry in $palette.ResourceMap.GetEnumerator()) {
+        $brush = [System.Windows.Media.SolidColorBrush]::new(
+            [System.Windows.Media.ColorConverter]::ConvertFromString($entry.Value))
+        $brush.Freeze()
+        $Window.Resources[$entry.Key] = $brush
+    }
+    $accentColor = [System.Windows.Media.ColorConverter]::ConvertFromString($palette.Accent)
+    foreach ($colorKey in 'SystemAccentColor','SystemAccentColorPrimary',
+            'SystemAccentColorSecondary','SystemAccentColorTertiary') {
+        $Window.Resources[$colorKey] = $accentColor
+    }
+
+    $Mode
+}
+
 function New-SnipThemeResources {
     [CmdletBinding()]
     param(