[IMP] web: patch BS4 to have beautiful colored cards

BS4 colored cards do not have a very popular design. This will reset
them to a BS3-like one: only the header and footer are colored and the
body will use the color of a default card background with a light
opacity (to let the chosen color appear).

addons/web/static/src/scss/bootstrap_review.scss

@@ -8,3 +8,10 @@
     // They would go over floating elements, which is never what we want.
     clear: both;
 }
+
+.card-body {
+    // BS4 colored cards do not have a very popular design. This will reset them
+    // to a BS3-like one: only the header and footer are colored and the body
+    // will use the color of a default card background with a light opacity.
+    @include o-bg-color(rgba($card-bg, $o-card-body-bg-opacity));
+}

addons/web/static/src/scss/primary_variables.scss

@@ -85,6 +85,8 @@ $o-cp-breadcrumb-height: 30px;
 
 $o-datepicker-week-color: #8f8f8f;
 
+$o-card-body-bg-opacity: 0.9;
+
 // Kanban
 
 $o-kanban-default-record-width: 300px;