Use better colors for activity calendar in darkmode

Changelog: changed
Signed-off-by: Janis Altherr <jaltherr@gitlab.com>

app/assets/stylesheets/framework/variables.scss

@@ -720,7 +720,7 @@ $calendar-activity-colors: (
   #7fa8c9,
   #527ba0,
   #254e77,
-);
+) !default;
 
 /*
 * Commit Page

app/assets/stylesheets/themes/_dark.scss

@@ -259,3 +259,11 @@ $line-removed-dark: $red-200;
 
 $well-expand-item: $gray-200;
 $well-inner-border: $gray-200;
+
+$calendar-activity-colors: (
+  #303030,
+  #333861,
+  #4a5593,
+  #6172c5,
+  #788ff7
+);