[css-text-4] Fix examples in dark mode

Closes #11099

Author: frivoal

css-text-4/Overview.bs

@@ -1196,6 +1196,7 @@ Expanding Between Words: the 'word-space-transform' property</h3>
 			display: block;
 			margin: auto;
 			border: solid gray 1px;
+			color: black;
 			background: white;
 		}
 		</style>
@@ -1261,6 +1262,7 @@ Expanding Between Words: the 'word-space-transform' property</h3>
 			display: block;
 			margin: auto;
 			border: solid gray 1px;
+			color: black;
 			background: white;
 			text-align: left;
 		}
@@ -6882,6 +6884,7 @@ Breaking Rules for Letters: the 'word-break' property</h3>
 				display: block;
 				margin: auto;
 				border: solid gray 1px;
+				color: black;
 				background: white;
 			}
 		</style>