[FIX] web_editor: fix switch design (website publish btn)

Using chrome, the design was not pixel perfect (letting a white bar
appear around the switch). Replacing the linear gradient color by a
simple color fixes that particular problem. The design is still not
pixel perfect but those are meant to be replaced in master anyway.

Part of https://github.com/odoo/odoo/pull/31117
task-1941156

addons/web_editor/static/src/scss/web_editor.ui.scss

@@ -1086,7 +1086,7 @@ body.editor_enable {
 
         &:checked + span {
             box-shadow: none;
-            background-image: linear-gradient(0deg, $o-we-color-success, darken($o-we-color-success, 10%));
+            background: $o-we-color-success;
 
             &:after {
                 margin-left: ($o-we-switch-size*1.8 - $o-we-switch-size) + 0.1;
@@ -1097,7 +1097,7 @@ body.editor_enable {
         > input {
             &:not(:checked) + span {
                 box-shadow: none;
-                background-image: linear-gradient(0deg, lighten(theme-color('danger'), 5%), darken(theme-color('danger'), 5%));
+                background: $o-we-color-danger;
             }
         }
     }