Update first chapter of the Theme documentation

Author: Julien Neuhart

docs/docs/15_Theme/1_BootstrapVue.md docs/docs/15_Theme/1_Customization.mddocs/docs/15_Theme/1_BootstrapVue.md renamed to docs/docs/15_Theme/1_Customization.md

@@ -1,14 +1,28 @@
 ---
-title: BootstrapVue
-slug: /theme/bootstrap-vue
+title: Customization
+slug: /theme/customization
 ---
 
+## Application Name
+
+Both the `webapp` and `api` services read the `APP_NAME` environment variable.
+
+Its value comes from the root *.env* file.
+
+If you update this value, you will have to restart these services (locally by doing `make down up`).
+
+:::note
+
+📣  Don't forget to update the file *.env.dist* if this change is definitive.
+
+:::
+
+## BootstrapVue
+
 The Symfony Boilerplate uses [BootstrapVue](https://bootstrap-vue.org/) as templating framework.
 
 Of course, you may use another templating framework, although it will require some extra work.
 
-## Add Plugin
-
 Most of the components from [BootstrapVue](https://bootstrap-vue.org/) come in the form of plugins.
 
 You have to add them manually in the *src/webapp/nuxt.config.js* file:
@@ -27,15 +41,23 @@ bootstrapVue: {
 
 ## Change Colors
 
+### Pages
+
 The file *src/webapp/assets/css/_variables.scss* is where you may define most of the colors.
 
 A few other `.scss` files also contain colors definition.
 
-For the Nuxt.js loading bar, its color is configurable in the  *src/webapp/nuxt.config.js* file:
+### Loading Bar
+
+Its color is configurable in the  *src/webapp/nuxt.config.js* file:
 
 ```js title="src/webapp/nuxt.config.js"
 loading: {
     color: '#a211fa',
     height: '3px',
 },
-``` 
+```
+
+### Emails
+
+The file *src/api/assets/css/emails.css* is where you may define most of the colors.