docs: replace background with background-color for consistency (sveltejs#467)

Fixes potential confusion by the tutorial using `background-color`
to overwrite `background` and referencing `background-color`
instead of the shorthand `background` used by the app.

Author: baderj

content/tutorial/02-advanced-svelte/06-classes-and-styles/04-component-styles/README.md

@@ -37,7 +37,7 @@ Inside `Box.svelte`, change `background-color` so that it is determined by a [CS
 		height: 5em;
 		border-radius: 0.5em;
 		margin: 0 0 1em 0;
-		background: +++var(--color, #ddd)+++;
+		background-color: +++var(--color, #ddd)+++;
 	}
 </style>
 ```
@@ -55,4 +55,4 @@ Any parent element (such as `<div class="boxes">`) can set the value of `--color
 
 The values can be dynamic, like any other attribute.
 
-This feature works by wrapping each component in a `<div style="display: contents">`, where needed, and applying the custom properties to it.
+This feature works by wrapping each component in a `<div style="display: contents">`, where needed, and applying the custom properties to it.

content/tutorial/02-advanced-svelte/06-classes-and-styles/04-component-styles/app-a/src/lib/Box.svelte

@@ -6,6 +6,6 @@
 		height: 5em;
 		border-radius: 0.5em;
 		margin: 0 0 1em 0;
-		background: #ddd;
+		background-color: #ddd;
 	}
-</style>
+</style>

content/tutorial/02-advanced-svelte/06-classes-and-styles/04-component-styles/app-b/src/lib/Box.svelte

@@ -6,6 +6,6 @@
 		height: 5em;
 		border-radius: 0.5em;
 		margin: 0 0 1em 0;
-		background: var(--color, #ddd);
+		background-color: var(--color, #ddd);
 	}
-</style>
+</style>