[FIX] mass_mailing: properly allow global customize separator styles

This is an addition to commit 098202ba584ca6ba3544ccd00397578f5deb12ad
The design tab allows the customization of the separators' border-style
and width as well as color, but this is currently broken because it gets
overridden by the default styles of the snippet, which has higher
specificity.

Part-of: odoo/odoo#95940

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

@@ -28,7 +28,7 @@ export const PRIORITY_STYLES = {
     'a:not(.btn)': [],
     'a.btn.btn-primary': [],
     'a.btn.btn-secondary': [],
-    'hr': ['border-top-color'],
+    'hr': ['border-top-width','border-top-style','border-top-color'],
 };
 export const RE_CSS_TEXT_MATCH = /([^{]+)([^}]+)/;
 export const RE_SELECTOR_ENDS_WITH_GT_STAR = />\s*\*\s*$/;