[FIX] web_editor: restore white color for discard button

Fixing BS behavior in https://github.com/odoo/odoo/commit/e24c3409d915ed73d86077b723b6e0cd77134e80
broke that button color as the button bg was set to transparent, so the
text color was computed to dark instead of light (as it should).

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

@@ -62,11 +62,7 @@
                 @include button-variant($o-brand-odoo, $o-brand-odoo);
             }
             &.btn-secondary {
-                @include button-variant(rgba(0, 0, 0, 0), rgba(0, 0, 0, 0));
-
-                &:hover {
-                    background: rgba(black, 0.5);
-                }
+                @include button-variant($o-we-color-dark, $o-we-color-dark);
             }
 
             &:focus, &:active, &:focus:active {