Merge branch 'dz-update-outdated-colors' into 'master'

Replace outdated bluish colors with default bg color

cc @skyruler 

See merge request !3973

app/assets/stylesheets/framework/blocks.scss

 .light-well {
-  background-color: #f8fafc;
+  background-color: $background-color;
   padding: 15px;
 }
 

app/assets/stylesheets/framework/tw_bootstrap_variables.scss

@@ -153,8 +153,8 @@ $nav-link-padding: 13px $gl-padding;
 //== Code
 //
 //##
-$pre-bg:           #f4f7fd !default;
+$pre-bg:           $background-color !default;
 $pre-color:        $gl-gray !default;
-$pre-border-color: #e7e9ed;
+$pre-border-color: $border-color;
 
 $table-bg-accent: $background-color;