Transparent titlebar in fullscreen on Windows 7

### Describe the bug

When put in fullscreen, the tabs bar is completely transparent, showing content from the webpage behind

### Expected behavior

The background should be opaque.

### 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

System default: Windows 7, with Aero and transparency enabled.
Others: This theme from Mozilla Addons: https://addons.mozilla.org/en-US/firefox/addon/dark-theme-for-firefox/

### `user.js` setup

Default user.js used

### Additional context

_No response_

Start by reproducing fullscreen on Windows 7 with the photon style and inspect the SCSS controlling the fullscreen tabs bar. Confirm the fix by verifying that the bar has an opaque background and no webpage content shows through.

Written by the indexing model from the issue text.