Fix grid selection highlight with custom QSS themes

Extract selection-background-color from QSS and apply it to the
app palette so the grid's custom-painted highlight matches the theme.

booru_viewer/gui/app.py

@@ -1639,7 +1639,16 @@ def run() -> None:
         try:
             # Use Fusion style so QSS has full control over rendering
             app.setStyle("Fusion")
-            app.setStyleSheet(custom_css.read_text())
+            css_text = custom_css.read_text()
+            app.setStyleSheet(css_text)
+            # Extract selection color from QSS and apply to palette
+            import re
+            m = re.search(r'selection-background-color\s*:\s*(#[0-9a-fA-F]{3,8})', css_text)
+            if m:
+                from PySide6.QtGui import QPalette, QColor
+                pal = app.palette()
+                pal.setColor(QPalette.ColorRole.Highlight, QColor(m.group(1)))
+                app.setPalette(pal)
         except Exception:
             pass
 

booru_viewer/gui/grid.py

@@ -67,6 +67,8 @@ class ThumbnailWidget(QWidget):
         self.update()
 
     def paintEvent(self, event) -> None:
+        # Ensure QSS is applied so palette picks up custom colors
+        self.ensurePolished()
         p = QPainter(self)
         p.setRenderHint(QPainter.RenderHint.Antialiasing)
         pal = self.palette()