diff --git a/addons/mass_mailing/views/snippets_themes.xml b/addons/mass_mailing/views/snippets_themes.xml
index 65dc66185cb..1cf3516a5c2 100644
--- a/addons/mass_mailing/views/snippets_themes.xml
+++ b/addons/mass_mailing/views/snippets_themes.xml
@@ -11,7 +11,7 @@
                 <i class="fa fa-lg fa-mobile"/>
             </button>
             <button class="o_fullscreen_btn btn btn-primary">
-                <img src="/web_editor/font_to_img/61541/rgb(255,255,255)/16" alt="Fullscreen"/>
+                <img class="img-fluid" src="/web_editor/font_to_img/61541/rgb(255,255,255)/16" alt="Fullscreen"/>
             </button>
         </div>
     </xpath>