Bug 1743027 - Simplify plaintext.css dark mode code. r=jwatt

Now that we have dark color-scheme everywhere we don't need to hard-code
this.

Differential Revision: https://phabricator.services.mozilla.com/D132187

layout/style/res/plaintext.css

@@ -18,17 +18,8 @@ html:not([dir]) pre { /* Not a UA sheet, so doesn't use :-moz-has-dir-attr */
 }
 
 @-moz-document unobservable-document() {
-  @media (prefers-color-scheme: dark) {
   :root {
-      /**
-       * in-content-page-{color, background} for dark theme.
-       *
-       * TODO(emilio): Make this just `color-scheme: dark` once color-scheme ships,
-       * (or `color-scheme: light dark` outside the media query).
-       */
-      background: rgb(28,27,34);
-      color: rgb(251,251,254);
-    }
+    color-scheme: light dark;
   }
 }
 

toolkit/themes/shared/in-content/common.inc.css

@@ -119,7 +119,6 @@
   :root {
     /* Keep these in sync with:
      *
-     *  * plaintext.css
      *  * nsXPLookAndFeel::GenericDarkColor
      *  * The default value of browser.display.foreground_color.dark and
      *    browser.display.background_color.dark