diff --git a/src/agenda/agenda.cpp b/src/agenda/agenda.cpp
--- a/src/agenda/agenda.cpp
+++ b/src/agenda/agenda.cpp
@@ -1520,6 +1520,22 @@
     drawContents(&p, 0, -y(), d->mGridSpacingX * d->mColumns, d->mGridSpacingY * d->mRows + y());
 }
 
+/* compute black or white contrast color
+see https://stackoverflow.com/questions/1855884/determine-font-color-based-on-background
+-color
+*/
+static QColor contrastColor(const QColor &color)
+{
+    // Counting the perceptive luminance - human eye favors green color...
+    double luminance = ((0.299 * color.red()) + 0.587 * color.green() + 0.114 * color.blue()) / 255;
+
+    if (luminance > 0.5) {
+        return Qt::black;
+    } else {
+        return Qt::white;
+    }
+}
+
 /*
   Draw grid in the background of the agenda.
 */
@@ -1660,20 +1676,32 @@
         }
     }
 
+    // Compute the grid line color for both the hour and half-hour
     QPen hourPen;
     QPen halfHourPen;
 
     if (!d->preferences()->useSystemColor()) {
-        hourPen = d->preferences()->agendaGridBackgroundColor().darker(150);
-        halfHourPen = d->preferences()->agendaGridBackgroundColor().darker(125);
+        // compute a black or white contrasting color against the background color
+        hourPen = contrastColor(d->preferences()->agendaGridBackgroundColor());
+        // now lighten or darken just to soften the look a bit
+        if (hourPen.color() == Qt::black) {
+            // dark grey lines
+            halfHourPen = hourPen.color().lighter(500);
+            hourPen = hourPen.color().lighter(200);
+        } else {
+            // light grey lines
+            halfHourPen = hourPen.color().darker(200);
+            hourPen = hourPen.color().darker(150);
+        }
     } else {
         const QColor windowTextColor = palette().color(QPalette::WindowText);
+        // lighten or darken the window text color
         if (windowTextColor.red() + windowTextColor.green() + windowTextColor.blue() < (256 / 2 * 3)) {
-            // dark grey line
+            // dark grey lines
             hourPen = windowTextColor.lighter(200);
             halfHourPen = windowTextColor.lighter(500);
         } else {
-            // light grey line
+            // light grey lines
             hourPen = windowTextColor.darker(150);
             halfHourPen = windowTextColor.darker(200);
         }