[FIX] mass_mailing: ensure close button is visible in mobile preview

This commit adapts the style of the close button on the mass mailing "mobile
preview" to ensure that it's visible in both regular mode and dark mode.

Task-3326846

closes odoo/odoo#121796

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>

addons/mass_mailing/static/src/scss/mass_mailing_mobile_preview.scss

@@ -18,7 +18,7 @@
                 font-family: $o-we-font-family;
 
                 .btn-close {
-                    filter: invert(1) grayscale(100%) brightness(200%);
+                    filter: invert(50%) grayscale(100%) brightness(200%);
                 }
 
                 h4 {