[REF] web: Overwrite bts4 custom checkbox design

Overwrite bts4 custom-checkbox design (un-checked state only).
Use outline rule to avoid conflics with :focus and :hover states.
The change has been added in ui.scss to affect both backend and frontend.

addons/web/static/src/scss/ui.scss

@@ -45,3 +45,13 @@
         transform: rotate(180deg);
     }
 }
+
+// Overwrite bts4 custom-checkbox design (un-checked state only)
+.custom-control.custom-checkbox {
+    .custom-control-input:not(:checked):not(:indeterminate) {
+        ~ .custom-control-label:before {
+            background: none;
+            outline: 1px solid $o-main-text-color;
+        }
+    }
+}