Lower background brightness for considering a theme dark to 50.

Bug: 994548
Change-Id: I89776a8ff3ebec8adabc3d133dee62e50134ed33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762474
Auto-Submit: Aran Gilman <gilmanmh@google.com>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689389}

third_party/blink/renderer/core/accessibility/apply_dark_mode.cc

@@ -17,7 +17,7 @@ namespace blink {
 namespace {
 
 const int kAlphaThreshold = 100;
-const int kBrightnessThreshold = 128;
+const int kBrightnessThreshold = 50;
 
 // TODO(https://crbug.com/925949): Add detection and classification of
 // background image color. Most sites with dark background images also have a