[Linux] Brave does not respect system light/dark theme

Description

On a Linux system, Brave has its own controls for light/dark mode, but it does not respect the system’s overall theme. If the option to use the “Gtk” theme is chosen, the Brave UI does respect the dark mode, however it doesn’t set the prefers-color-scheme media query. This is a bug inherited from Chromium, and is being tracked here: https://bugs.chromium.org/p/chromium/issues/detail?id=998903

I know this should be fixed upstream, but Google doesn’t seem to care about it enough to justify the effort. One of the latest comments mentions a patch that a user has made, which solves the issue. The patch can be seen here in case a similar solution is intended.

Steps to Reproduce

1. Open Brave on a Linux system with light theme enabled
2. Open a website that supports prefers-color-scheme query, such as DuckDuckGo
3. Change system theme to dark mode
4. Website does not change theme

Actual result:

This is how Brave and other Chromium browsers behave (left), and how Firefox does (right). Note how in Chromium, the page content doesn’t change

Expected result:

Brave/Chromium (left), Firefox (right)

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.21.74 Chromium: 89.0.4389.72 (Official Build) (64-bit)
Revision	3f345f156bfd157bd1bea06310e55f3fb2490359-refs/branch-heads/4389@{#1393}
OS	Linux

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Haven’t tried, but no reason to believe it doesn’t happen there

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? Not related
• Does the issue resolve itself when disabling Brave Rewards? Not related
• Is the issue reproducible on the latest version of Chrome? Yes

Miscellaneous Information:

Latest versions of Ubuntu have a quick Light/Dark mode toggle so it should be really easy to test

[jmbuhr]

I can confirm this issue on my system (pop-os / ubuntu). While you can set the theme to GTK+, which matches the system theme for the window and titlebar, the actual brave colors only have the options Light and Dark and not match system theme as available on windows and MacOS.

[satrinity402]

Still an issue for me. Running Pop OS 21.10