Add a light/dark toggle to the Qt6 top bar

modules/ui/PySide6TopBarView.py

@@ -4,9 +4,9 @@
 from modules.ui.TopBarController import TopBarController
 from modules.util.enum.ModelType import ModelType
 from modules.util.enum.TrainingMethod import TrainingMethod
-from modules.util.ui import pyside6_components
+from modules.util.ui import pyside6_components, theme
 
-from PySide6.QtWidgets import QFileDialog, QWidget
+from PySide6.QtWidgets import QApplication, QFileDialog, QPushButton, QWidget
 
 
 class PySide6TopBarView(BaseTopBarView, QWidget):
@@ -33,6 +33,20 @@ def __init__(
         self.build(self.frame, master, controller, ui_state,
                    change_model_type_callback, change_training_method_callback, load_preset_callback)
 
+        # Theme switch. Lives in the PySide6 view, not the shared BaseTopBarView: the ctk top bar
+        # has no color scheme to switch. A text glyph follows the palette's ButtonText.
+        self.theme_button = pyside6_components.button(
+            self.frame, 0, 8, "◐", self.__toggle_theme,
+            tooltip="Switch between light and dark mode", sticky="v",
+        )
+        # Square and icon-sized: the grid stretches a button to its column otherwise. The side
+        # comes from a text-less button so the glyph's own metrics don't set it.
+        side = QPushButton().sizeHint().height()
+        self.theme_button.setFixedSize(side, side)
+
+    def __toggle_theme(self):
+        theme.apply_theme(QApplication.instance(), dark=not theme.is_dark_theme)
+
     def _setup_frame_column_weight(self):
         pyside6_components._layout(self.frame).setColumnStretch(5, 1)
 

modules/util/ui/pyside6_util.py

@@ -3,8 +3,8 @@
 import sys
 from abc import ABCMeta
 
-from PySide6.QtCore import Qt
-from PySide6.QtGui import QColor, QPalette
+from modules.util.ui.theme import apply_theme
+
 from PySide6.QtWidgets import QApplication, QStyleFactory, QWidget
 
 
@@ -32,36 +32,7 @@ def create_application() -> QApplication:
     # controls via OS theme APIs, which breaks once an application stylesheet
     # is set, producing a flatter look than Fusion's own stylesheet-aware painting.
     app.setStyle(QStyleFactory.create("Fusion"))
-    app.styleHints().setColorScheme(Qt.ColorScheme.Light)
-
-    palette = app.palette()
-    palette.setColor(QPalette.ColorRole.Base, QColor("white"))
-    palette.setColor(QPalette.ColorGroup.Disabled, QPalette.ColorRole.Base, QColor("#e0e0e0"))
-    app.setPalette(palette)
 
-    app.setStyleSheet("""
-        QLineEdit, QSpinBox, QDoubleSpinBox, QTextEdit, QPlainTextEdit {
-            padding: 2px 2px;
-        }
-        QCheckBox::indicator {
-            width: 16px;
-            height: 16px;
-        }
-        QProgressBar {
-            background-color: #c8c8c8;
-        }
-        QToolButton {
-            padding-top: 0px;
-            padding-bottom: 0px;
-            padding-right: 40px;
-        }
-        QToolButton::menu-indicator {
-            subcontrol-origin: padding;
-            subcontrol-position: right center;
-            width: 12px;
-            height: 12px;
-            right: 10px;
-        }
-    """)
+    apply_theme(app)
 
     return app

modules/util/ui/theme.py

@@ -0,0 +1,69 @@
+import platform
+
+from PySide6.QtCore import Qt
+from PySide6.QtGui import QColor, QPalette
+from PySide6.QtWidgets import QApplication
+
+IS_WINDOWS = platform.system() == "Windows"
+
+_BASE_STYLESHEET = """
+    QLineEdit, QSpinBox, QDoubleSpinBox, QTextEdit, QPlainTextEdit {
+        padding: 2px 2px;
+    }
+    QCheckBox::indicator {
+        width: 16px;
+        height: 16px;
+    }
+    QProgressBar {
+        background-color: #c8c8c8;
+    }
+    QToolButton {
+        padding-top: 0px;
+        padding-bottom: 0px;
+        padding-right: 40px;
+    }
+    QToolButton::menu-indicator {
+        subcontrol-origin: padding;
+        subcontrol-position: right center;
+        width: 12px;
+        height: 12px;
+        right: 10px;
+    }
+"""
+
+# A scheme change regenerates only the palette roles nobody set explicitly. Base is set below
+# for light mode, so it would keep that white through every later switch; each scheme is applied
+# from a pristine copy taken before the first override instead.
+_scheme_palettes = {}
+
+# Whether the dark palette is the one currently applied.
+is_dark_theme = False
+
+
+def _capture_scheme_palettes(app: QApplication) -> None:
+    original_scheme = app.styleHints().colorScheme()
+    for scheme in (Qt.ColorScheme.Light, Qt.ColorScheme.Dark):
+        app.styleHints().setColorScheme(scheme)
+        _scheme_palettes[scheme] = QPalette(app.palette())
+    app.styleHints().setColorScheme(original_scheme)
+
+
+def apply_theme(app: QApplication, dark: bool | None = None) -> None:
+    global is_dark_theme
+
+    if not _scheme_palettes:
+        _capture_scheme_palettes(app)
+
+    if dark is None:
+        is_dark =  app.palette().color(QPalette.ColorRole.Window).lightness() < 128
+        dark = IS_WINDOWS and is_dark
+    is_dark_theme = dark
+
+    scheme = Qt.ColorScheme.Dark if dark else Qt.ColorScheme.Light
+    app.styleHints().setColorScheme(scheme)
+    palette = QPalette(_scheme_palettes[scheme])
+    if not dark:
+        palette.setColor(QPalette.ColorRole.Base, QColor("white"))
+        palette.setColor(QPalette.ColorGroup.Disabled, QPalette.ColorRole.Base, QColor("#e0e0e0"))
+    app.setPalette(palette)
+    app.setStyleSheet(_BASE_STYLESHEET)