[Spreadsheet] Use stylesheet for alias color

When setting the text color for an invalid alias, use the Qt stylesheeet
mechanism rather than QPalette, so that it works correctly with
an applied QSS stylesheeet. Also attempt to detect a darkmode stylesheet
and use lighter shade of red so that the alias is more legible. Finally,
instead of explicitly setting the text color to black when it's valid,
reset to the original stylesheet (usually an empty string). This ensures
that in a dark stylesheet the text color is legible.

Fixes #4803.

src/Mod/Spreadsheet/Gui/SpreadsheetView.cpp

@@ -419,24 +419,27 @@ void SheetView::aliasChanged(const QString& text)
     // check live the input and highlight if the user input invalid characters
 
     bool aliasOk = true;
-    QPalette palette = ui->cellAlias->palette();
+    static auto originalStylesheet = ui->cellAlias->styleSheet();
+    QString warningColor;
+    if (qApp->styleSheet().contains(QLatin1String("dark"), Qt::CaseInsensitive))
+        warningColor = QLatin1String("rgb(255,90,90)"); // Light red for dark mode
+    else
+        warningColor = QLatin1String("rgb(200,0,0)"); // Dark red for light mode
 
     if (!text.isEmpty() && !sheet->isValidAlias(Base::Tools::toStdString(text)))
         aliasOk = false;
 
     if (!aliasOk) {
         // change tooltip and make text color red
         ui->cellAlias->setToolTip(QObject::tr("Alias contains invalid characters!"));
-        palette.setColor(QPalette::Text, Qt::red);
+        ui->cellAlias->setStyleSheet(QLatin1String("color:") + warningColor);
     }
     else {
         // go back to normal
         ui->cellAlias->setToolTip(
             QObject::tr("Refer to cell by alias, for example\nSpreadsheet.my_alias_name instead of Spreadsheet.B1"));
-        palette.setColor(QPalette::Text, Qt::black);
+        ui->cellAlias->setStyleSheet(originalStylesheet);
     }
-    // apply the text color via the palette
-    ui->cellAlias->setPalette(palette);
 }
 
 void SheetView::currentChanged ( const QModelIndex & current, const QModelIndex & previous  )