Bug 1710643 - Use an slightly higher threshold to consider a color dark. r=Gijs

Luminance goes from 0 to 255, so using 127 makes sense, and allows all
disabled titlebar colors that I found in various GTK themes to still be
considered dark enough (for those, 110 was too low).

Differential Revision: https://phabricator.services.mozilla.com/D114876

browser/base/content/browser.js

@@ -8783,8 +8783,9 @@ var ToolbarIconColor = {
       luminances.set(toolbar, luminance);
     }
 
+    const luminanceThreshold = 127; // In between 0 and 255
     for (let [toolbar, luminance] of luminances) {
-      if (luminance <= 110) {
+      if (luminance <= luminanceThreshold) {
         toolbar.removeAttribute("brighttext");
       } else {
         toolbar.setAttribute("brighttext", "true");

browser/modules/ThemeVariableMap.jsm

@@ -194,5 +194,5 @@ const ThemeContentPropertyList = [
 
 // This is copied from LightweightThemeConsumer.jsm.
 function _isColorDark(r, g, b) {
-  return 0.2125 * r + 0.7154 * g + 0.0721 * b <= 110;
+  return 0.2125 * r + 0.7154 * g + 0.0721 * b <= 127;
 }

toolkit/modules/LightweightThemeConsumer.jsm

@@ -480,5 +480,5 @@ function _rgbaToString(parsedColor) {
 
 // There is a second copy of this in ThemeVariableMap.jsm.
 function _isColorDark(r, g, b) {
-  return 0.2125 * r + 0.7154 * g + 0.0721 * b <= 110;
+  return 0.2125 * r + 0.7154 * g + 0.0721 * b <= 127;
 }