Bug 1898096 - Replace color settings with "contrast control". r=emilio,fluent-reviewers,settings-reviewers,accessibility-frontend-reviewers,morgan,bolsson,masayuki,mossop,pdfjs-reviewers,calixte

This patch changes the backing prefs by relying on the tristate offered
by browser.display.document_color_use instead of
browser.display.use_system_colors. This simplifies the color
decision tree, and offers a simplified UI.

The tristate preference offered to the user is as follows:
 1. "Use platform's contrast settings" (document_color_use=0)
 2. "Off" - never use HCM, regardless of platform setting (document_color_use=1)
 3. "On" - always use HCM, regardless of platform setting (document_color_use=2)

Option 3 also reveals a colors UI for the user to choose the palette the
browser HCM will use (bg/text/link/visited).

There are three color palettes to choose from in light of the preference
above they are:
 1. WIDGET_COLORS: The OS's configured colors used by its theme.
 2. HARDCODED_COLORS: Colors deemed as standard and are hard coded into
    Firefox (eg. white on black text, blue and purple links).
 3. PREFERENCE_COLORS: Colors that are stored in preferences and are
    configurable from the colors UI (eg. browser.visited_color and
    browser.display.background_color)

The decision over which palette to use is as follows:
 * If we are styling browser UI -> WIDGET_COLORS
 * else, if resist fingerprinting is enabled -> HARDCODED_COLORS
 * else, if document_color_use==0 AND OS HCM is on -> WIDGET_COLORS
 * else, if document_color_use==2 -> PREFERENCE_COLORS
 * else -> HARDCODED_COLORS

Differential Revision: https://phabricator.services.mozilla.com/D211115

browser/components/tabbrowser/content/tabbrowser.js

@@ -197,11 +197,9 @@
       if (
         Services.prefs.getIntPref("browser.display.document_color_use") == 2
       ) {
-        this.tabpanels.style.backgroundColor = Services.prefs.getBoolPref(
-          "browser.display.use_system_colors"
-        )
-          ? "canvas"
-          : Services.prefs.getCharPref("browser.display.background_color");
+        this.tabpanels.style.backgroundColor = Services.prefs.getCharPref(
+          "browser.display.background_color"
+        );
       }
 
       this._setFindbarData();