Created attachment 176320 [details]
Reproduction screencast (for `#c0`).

SUMMARY
-------

Currently, the Web Browser plasmoid and `khelpcenter` render `about:blank` and status code / error pages with a `#FFFFFF` background. At night, this can be startling if `plasma-apply-colorscheme` is set to `BreezeDark`.

[STEPS TO REPRODUCE][7]
-----------------------

- 1. Add the Web Browser plasmoid to `plasmashell`.
  2. - Navigate to `about:blank`.
     - Navigate to `localhost:0/`.
- Invoke `khelpcenter`.

[OBSERVED RESULT][3]
--------------------

The `HTML`'s `background-color` appears to be hard-coded as either of the undermentioned:

- ~~~CSS
  HTML { background-color: #FFFFFF; }
  ~~~

- ~~~CSS
  HTML { color-scheme: light; }
  ~~~

I doubt that `@media (prefers-color-scheme:` is involved.

[EXPECTED RESULT][4]
--------------------

As [Firefox permits using its `browser.display.use_system_colors` preference][1], `about:blank` in `khelpcenter` the Web Browser plasmoid should render usiung the system `.colors` scheme's background-attribute's colour value.

To *my* knowledge, this can be remdiated by *unsetting* `background-color`, and *setting* the undermentioned:

~~~CSS
HTML { color-scheme: light dark !important; }
~~~

However, this is merely a hypothesis.

[SOFTWARE/OS VERSIONS][2]
-------------------------

1. ~~~sh
   #!/usr/bin/env -S bash
   kinfo
   ~~~

2. > ~~~YAML
   > Operating System       : Fedora Linux 41
   > KDE Plasma Version     : 6.2.4
   > KDE Frameworks Version : 6.8.0
   > Qt Version             : 6.8.0
   > Kernel Version         : 6.11.10-300.fc41.x86_64 (64-bit)
   > Graphics Platform      : Wayland
   > Processors             : 12 × AMD Ryzen 5 7600X 6-Core Processor
   > Memory                 : 30.4 GiB of RAM
   > Graphics Processor     : AMD Radeon RX 5700
   > Manufacturer           : ASRock
   > Product Name           : X670E Taichi
   > ~~~

ADDITIONAL INFORMATION
----------------------

Nominally filed here, because it becomes increasingly difficult to file at [QtWebEngine][5] or [Chromium][6] if they don't believe that you've consulted downstream first.

[1]: https://superuser.com/revisions/1480115/2
[2]: https://stackoverflow.com/revisions/76446386/3
[3]: https://imgur.com/a/qgupNfQ
[4]: https://imgur.com/a/0LlkZSM
[5]: https://bugreports.qt.io/secure/CreateIssue!default.jspa
[6]: https://issues.chromium.org/issues/wizard
[7]: https://youtu.be/MClyv7Gx5r4