Updated styles of show invisibles (#1607)

Changes the color of the placeholders of invisible characters.
They will now use the default text color of the theme as the base
and blend in instead of a fixed color for all themes.

plugins/show-invisibles/prism-show-invisibles.css

@@ -9,25 +9,26 @@
 .token.cr:before,
 .token.lf:before,
 .token.space:before {
-	color: hsl(24, 20%, 85%);
+	color: inherit;
+	opacity: 0.4;
 	position: absolute;
 }
 
 .token.tab:not(:empty):before {
-    content: '\21E5';
+	content: '\21E5';
 }
 
 .token.cr:before {
-    content: '\240D';
+	content: '\240D';
 }
 
 .token.crlf:before {
-    content: '\240D\240A';
+	content: '\240D\240A';
 }
 .token.lf:before {
-    content: '\240A';
+	content: '\240A';
 }
 
 .token.space:before {
 	content: '\00B7';
-}
+}