Grey, opaque background on Windows 7 titlebar

### Describe the bug

Since version 8.6.3 (ESR edition), the background appears as opaque grey instead on the intended Aero background on Windows 7.
It even happens in custom themes, where Aero isn't even enabled. In my dark theme, which has a dark background, this gray rectangle appears instead of the intended dark grey, the same way seen in the screenshot above.

### Expected behavior

The intended behavior, seen up to 8.6.2, is a transparent, Aero background.

### Screenshots

_No response_

### OS

Windows 7

### OS - Others

_No response_

### Firefox Version

115.17.0esr (64-bits)

### Distribution

- [ ] [Original Lepton](https://github.com/black7375/Firefox-UI-Fix)
- [X] [Lepton's photon style](https://github.com/black7375/Firefox-UI-Fix/tree/photon-style)
- [ ] [Lepton's proton style](https://github.com/black7375/Firefox-UI-Fix/tree/proton-style)

### Theme

- [ ] Light
- [ ] Dark
- [ ] Alpenglow (System: Light)
- [ ] Alpenglow (System: Dark)
- [X] System Default
- [X] Others: (e.g. [Photon Colors](https://addons.mozilla.org/en-US/firefox/addon/photon-colors/), [Google Chrome Dark](https://addons.mozilla.org/en-US/firefox/addon/google-chrome-dark/), [Fairytale Of Nature](https://addons.mozilla.org/en-US/firefox/addon/fairytale-of-nature/))

### Theme - More Info

Issue also apparent on the following custom theme: https://addons.mozilla.org/en-US/firefox/addon/dark-theme-for-firefox/
For system default, Aero is enabled, as well as transparency.

### `user.js` setup

Default user.js used

### Additional context

_No response_

Start by reproducing the titlebar appearance on Windows 7 with Firefox 115.17.0esr, comparing the behavior reported for versions 8.6.2 and 8.6.3. Done means the titlebar again shows the transparent Aero background and does not replace custom-theme backgrounds with opaque grey.

Written by the indexing model from the issue text.