Fixed issues with line number colors on dark themes

By using the palette foreground color for line numbers when the
background is very dark, instead of the palette's "dark" color.

src/plugins/texteditor/fontsettingspage.cpp

@@ -143,10 +143,23 @@ QString FormatDescription::trName() const
 
 QColor FormatDescription::foreground() const
 {
-    if (m_name == QLatin1String(Constants::C_LINE_NUMBER))
-        return QApplication::palette().dark().color();
-    if (m_name == QLatin1String(Constants::C_PARENTHESES))
+    if (m_name == QLatin1String(Constants::C_LINE_NUMBER)) {
+        const QColor bg = QApplication::palette().background().color();
+        if (bg.value() < 128) {
+            return QApplication::palette().foreground().color();
+        } else {
+            return QApplication::palette().dark().color();
+        }
+    } else if (m_name == QLatin1String(Constants::C_CURRENT_LINE_NUMBER)) {
+        const QColor bg = QApplication::palette().background().color();
+        if (bg.value() < 128) {
+            return QApplication::palette().foreground().color();
+        } else {
+            return m_format.foreground();
+        }
+    } else if (m_name == QLatin1String(Constants::C_PARENTHESES)) {
         return QColor(Qt::red);
+    }
     return m_format.foreground();
 }