fix hover color of markdown toolbar buttons

www/kanban/app-kanban.less

@@ -95,6 +95,12 @@
         .cp-markdown-toolbar {
             background-color: #eee;
             color: @cryptpad_text_col;
+            button {
+                &:hover {
+                    background-color: @cryptpad_text_col;
+                    color: white;
+                }
+            }
         }
         margin-bottom: 15px;
     }