diff --git a/addons/mass_mailing/static/src/js/mass_mailing_html_field.js b/addons/mass_mailing/static/src/js/mass_mailing_html_field.js
index 614e62f17ec..9af8053cbd1 100644
--- a/addons/mass_mailing/static/src/js/mass_mailing_html_field.js
+++ b/addons/mass_mailing/static/src/js/mass_mailing_html_field.js
@@ -406,6 +406,8 @@ export class MassMailingHtmlField extends HtmlField {
             $target.parents('.o_mail_template_preview').remove();
         });
 
+        // Clear any previous theme class before adding new one.
+        this.wysiwyg.$iframeBody.closest('body').removeClass(this._themeClassNames);
         let selectedTheme = this._getSelectedTheme(themesParams);
         if (selectedTheme) {
             this.wysiwyg.$iframeBody.closest('body').addClass(selectedTheme.className);