ESR115 minimize maximize close buttons hidden on non-system theme.

### Describe the bug

The minimize, maximize, and close buttons are covered in non-system theme. (Dark/Light/Alpenglow etc). They still work, but are not visible.

### Expected behavior

The minimize, maximize, and close buttons should be visible.

### Screenshots

non-system
system

### OS

Linux

### OS - Others

_No response_

### Firefox Version

v115.16.0esr

### 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

- [X] Light
- [X] Dark
- [X] Alpenglow (System: Light)
- [X] Alpenglow (System: Dark)
- [ ] System Default
- [ ] 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

_No response_

### `user.js` setup

user.js setup

```javascript
// Your settings
```

### Additional context

Similar to https://github.com/black7375/Firefox-UI-Fix/issues/218 - but on Linux and on non-system themes only. DE is KDE and system theme is Adwaita/Breeze-Dark.
Non-ESR version seems to not be affected - tested with v130.

Reproduce the issue on Linux with KDE using Firefox ESR 115.16.0, the photon style, and a non-system theme, then compare it with the system theme shown in the screenshots. Trace the photon-style UI styling responsible for the window controls. Done means the minimize, maximize, and close buttons are visible and remain functional across the listed non-system themes.

Written by the indexing model from the issue text.