User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:88.0) Gecko/20100101 Firefox/88.0

Steps to reproduce:

Upgraded from firefox 88.0.1 to 89.0 under Windows 10 20H2 OS Build 19042.1023
Ran firefox, selected "System theme" at new theme prompt.

Actual results:

firefox title bar was very light (the same as firefox "Light" theme), rather than
the steel gray it was in firefox 88.0.1 (but window border was correct).

How to reproduce:
To best demonstrate this, set a distinctive title bar color (red will do nicely) as follows:
In Settings/Personalize (or right-click desktop/Personalize):
Go to "Colors" from list at left.
Scroll down to color pallets below "Choose your accent color"; select red.
Below "Show accent color on the following surfaces", check "Title bars and
window borders".
Run firefox; set theme to "System theme" (under Tools/Add-ons .../Themes).
The window border is red, as expected, but not the title bar (it's the same as the
firefox "Light" theme).

Expected results:

With red as the system title bar color, firefox title bar should be red when focussed.

The User Agent in the report shows Fedora, because that's where I composed the report, but the big occurred under Windows 10 20H2 OS Build 19042.102 .

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Gtk' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Is this an intentional change?

(In reply to Emilio Cobos Álvarez (:emilio) from comment #3)

Is this an intentional change?

Yes.

This bug does NOT occur under Windows 8.1 (x64) (build 9600.20018) (with all Windows updates to date).
"Troubleshoot Mode" make no difference in behavior under either Windows OS.

To re-iterate: The User Agent shown in the initial report is NOT from the OS under which the bug occurs. That User Agent is:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0