The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/500106c1e60fdc36dd3cb5af56c430a0c77dc2bb

commit 500106c1e60fdc36dd3cb5af56c430a0c77dc2bb
Author: Ryan Meier <rameier@chromium.org>
Date: Wed Oct 31 18:33:51 2018

Added incognito mappings for COLOR_WINDOW_CONTROL_BUTTON_BACKGROUND_* to theme_service

GetIncognitoId() (in theme_service.cc) was not providing the incognito variants of control button background colors, so the caption buttons were receiving the non-incognito colors to blend against - causing them to have insufficient contrast in incognito with certain themes.

Bug:  889711 
Change-Id: I801ed5b915d67e12b6e998c8d96400baf04c3e40
Reviewed-on: https://chromium-review.googlesource.com/c/1310497
Commit-Queue: Ryan Meier <rameier@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604344}
[modify] https://crrev.com/500106c1e60fdc36dd3cb5af56c430a0c77dc2bb/chrome/browser/themes/theme_service.cc

Ryan Meier@ Could you please provide manual reproducible steps which helps us in reproducing the issue and also please provide details of the theme that reproduces the issue and help us in verifying the fix.

Thanks!

Ah yes - of course, sorry about that!

1) Download the manifest.json file from the initial bug report, and load it as an unpacked extension.
2) Open an incognito window.
3) Click elsewhere so the incognito window loses focus.

The caption button glyphs (minimize, maximize, close) should be a ~medium gray, instead of black (as depicted in the screenshot attached to the initial bug report).

Ryan Meier@ Thanks for your feedback!

Ryan Meier@ Able to reproduce the issue on chrome version# 71.0.3561.0 and able to verify the fix on chrome version# 72.0.3602.2 using Windows-10. But, the buttons(minimize, maximize, close) rendering on Linux and Mac are different and it is not as per screenshot mentioned in comment# 0. Could you please confirm if this issue is applicable to Linux and Mac and help us in verifying the fix.

Thanks!

I think it's safe to say this issue was only applicable to Windows 10.  

Just to be sure - I did a quick check locally and it seems that the caption buttons that would display on Linux (or other versions of Windows) are working fine, and I don't believe themes even affect the caption buttons on Mac.