Added automatic listing of custom CSS sheets (no more hardcoded), added error message when current selection is not valid and select default theme

InvenTree/templates/InvenTree/settings/theme.html

@@ -18,6 +18,15 @@
     {% csrf_token %}
     {% load crispy_forms_tags %}
     {% crispy form %}
-</form> 
+</form>
+
+{% if invalid_color_theme %}
+	<div class="alert alert-danger alert-block" role="alert" style="display: inline-block;">
+	{% blocktrans %}
+		The CSS sheet "{{invalid_color_theme}}.css" for the currently selected color theme was not found.<br>
+		Please select another color theme :)
+	{% endblocktrans %}
+	</div>
+{% endif %}
 
 {% endblock %}