[FIX] mass_mailing: make sure #design-element is prepended to .o_layout

When styling a theme in XML, we don't have access to the top of
`.o_layout`, leading to the design element being located in different
places depending on whether it's been added programmatically or in XML.
This harmonizes it, ensuring it's always at the top of `.o_layout`.

X-original-commit: ab61f1d38b2852ec0b6727122a1cfc2cd81cfc9f
Part-of: odoo/odoo#92138

addons/mass_mailing/static/src/js/mass_mailing_design_constants.js

@@ -94,10 +94,10 @@ export const initializeDesignTabCss = $editable => {
             // If a style element can't be found, create one and initialize it.
             styleElement = document.createElement('style');
             styleElement.setAttribute('id', 'design-element');
-            // The style element needs to be within the layout of the email in
-            // order to be saved along with it.
-            $editable.find('.o_layout').prepend(styleElement);
         }
+        // The style element needs to be within the layout of the email in
+        // order to be saved along with it.
+        $editable.find('.o_layout').prepend(styleElement);
 };
 
 export const FONT_FAMILIES = [