fix: add white fill to architecture images

make all texts in the image visible in dark mode

Author: jouni

articles/getting-started/tutorial/project-setup.adoc

@@ -44,10 +44,12 @@ A component is a grouping of related functionality encapsulated behind a well-de
 
 The C4 model doesn't require you to use all four levels. The application you are going to build during this tutorial is quite simple. You can jump straight to the container diagram, which looks like this:
 
+[.fill.white]
 image::images/container-diagram.png[]
 
 The application consists of two containers: the user's web browser; and a Spring Boot web application. Regardless of whether you build the user interface with Hilla or Flow, the containers remain the same. However, if you zoom in on them, you'll see the components are slightly different. The web application container looks like this:
 
+[.fill.white]
 image::images/component-diagram.png[]
 
 The application layer consists of two system components: _Chat_; and _In-memory Storage_. The chat component exposes an Application Programming Interface (API) for the user interface to call, and a Service Provider Interface (SPI) for storing and retrieving messages. This makes it possible to plug in different storage mechanisms without changing the chat component itself. In this tutorial, all chat messages are stored in memory. However, you could easily plug in another component that instead stores the messages in, for example, a relational database. This way of splitting systems into smaller components with clearly defined interfaces (i.e., APIs and SPIs) is a good praxis when building Vaadin applications and can be applied to both the application layer and the presentation layer.