Tested in chrome # 61.0.3163.79 and Canary #63.0.3212.0 on win 10.0 and not able to reproduce the issue.Please find the screen shots for your reference.

@davidmcgregor: Could you please let me know if i have missed anything and if possible,Please create new profile without extensions and apps.Re-check once and let us know the observations of the issue which would help us to triage the issue further.

Thanks in Advance.

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Yes I am able to reproduce.
Javascript files are fine, please re-try with a server response.
Set up a simple php server and just echo "Hello World" 
or do a var_dump; die();

Thank you for providing more feedback. Adding requester "rbasuvula@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Please also note, this is a response to an XHR request not a javascript file or other files.

Okay perfect I am able to reproduce easily:

Stick this into an index.php file in some test folder:

<?php
header("Content-Type: text/html; charset=UTF-8");
header('HTTP/1.1 500 Internal Server Error');

$myArray = [1, 2, 3, 4, "TEST"];

var_dump($myArray);die();


Then navigate to the folder and (with php installed) run php -S localhost:8000

You will be able to see that the text is both black and unformatted in your dev tools preview tab.

Or even easier:

http://htmledit.squarefree.com/testes

Open dev tools and check out preview text. It will be in black

Thanks for the update.

Able to reproduce the issue on Mac Os 10.12.6 , ubuntu 14.04 and windows 7 using M61 #61.0.3163.79 and M63 #63.0.3163.3 using the link given in commnet #7.

This issue is seen from from M50 and is a non-regression issue.

Marking it as untraiged for further inputs on this.

Thanks!

Any update? Frustrating that it's been this way for so long. It seems like a simple fix...

 Issue 855318  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c67340779f137602c4cb91ef5bfbaebc63226eed

commit c67340779f137602c4cb91ef5bfbaebc63226eed
Author: Joel Einbinder <einbinder@chromium.org>
Date: Wed Oct 10 18:03:49 2018

DevTools: Network html preview should always have a white background

The iframe doesnt change colors due to dark theme, so the background
needs to stay white as well. Otherwise there is dark text on a
dark background.

Bug:  763919 
Change-Id: I8a542dc613044bd8ac3d18dfccec66b65010ba0e
Reviewed-on: https://chromium-review.googlesource.com/c/1271943
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598399}
[modify] https://crrev.com/c67340779f137602c4cb91ef5bfbaebc63226eed/third_party/blink/renderer/devtools/front_end/network/requestHTMLView.css

Wait, the solution is just not to make the dark theme apply to the Preview Tab...?

Able to reproduce the issue on chrome version 61.0.3163.79(build withtout fix)as per the comment #0.
Verified the fix on Mac 10.13.6, Windows 10 and Ububntu 14.04. using Chrome version # 71.0.3577.0 .
Attaching screen-cast for reference.
Observed that  "Text white and lighter" 
The fix is working as expected, adding Verified labels

Thanks...!

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