LibWeb: Use -libweb-palette-base-text for text color in default CSS

This makes un-styled text readable when using a dark system theme,
previously such text would be black, regardless of the theme background
color.

Fixes #7274.

Userland/Libraries/LibWeb/CSS/Default.css

@@ -1,5 +1,6 @@
 html {
     font-family: sans-serif;
+    color: -libweb-palette-base-text;
 }
 
 a {