fix(ui): make alerts brighter (#56)

src/invenflask/templates/base.html

@@ -66,7 +66,7 @@
         {% with messages = get_flashed_messages(with_categories=true) %}
             {% if messages %}
             {% for category, message in messages %}
-                <div class="alert alert-{{ category }}"> {{ message }} </div>
+                <div class="alert bg-{{ category }}"> {{ message }} </div>
             {% endfor %}
             {% endif %}
         {% endwith %}