[css-ui][editorial] improve dark mode rendering of the spec

Author: frivoal

css-ui-4/Overview.bs

@@ -1622,7 +1622,7 @@ Animation of the insertion caret: 'caret-animation'</h4>
 		<style>
 		@keyframes caret-alternate-ref { 50% { border-color: red; } }
 		</style>
-		<div style="border:inset; background: white; width: 10em;">
+		<div style="border:inset; background: white; color: black; width: 10em;">
 			Text area
 			with color-alternating caret<span style="border-right: 2px solid blue; animation: caret-alternate-ref 2s step-end infinite;"></span>
 		</div>
@@ -1634,7 +1634,7 @@ Animation of the insertion caret: 'caret-animation'</h4>
 		}
 		</style>
 		<div contentEditable=true
-		     style="border:inset; background: white; width: 10em;
+		     style="border:inset; background: white; color: black; width: 10em;
 		            caret-animation: manual;
 		            caret-color: blue;
 		            animation: caret-alternate-test 2s step-end infinite;"
@@ -1788,6 +1788,7 @@ Shape of the insertion caret: 'caret-shape'</h4>
 		}
 		#caret-shape-example td+td {
 			background:white;
+			color: black;
 		}
 		</style>
 		<table id="caret-shape-example">