Light theme is broken in Firefox 146

### Describe the bug

Light theme is no longer "white" in Firefox 146. Title bar/tabs bar, context menu and bunch of other elements now grey instead of white.

There is no difference between Light and System theme - auto in Firefox 146 (and 147 beta), so maybe this is Firefox bug? Or, rather, this is fixed bug and this is how Firefox should look like in Windows 10 by default?

### Expected behavior

Same as in Firefox 145 and earlier, light theme should be "white" instead of "grey".

### Screenshots

How it looks now in Firefox 146 (portable version, clean profile, default settings):

How it looked in Firefox 145 (portable version, clean profile, default settings):

### OS

Windows 10

### OS - Others

_No response_

### Firefox Version

146.0

### 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
- [ ] Dark
- [ ] Alpenglow (System: Light)
- [ ] 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
// Default settings
```

### Additional context

_No response_

Reproduce the light-theme appearance on Windows 10 in Firefox 146, comparing it with Firefox 145 using a clean portable profile and the photon-style distribution. Trace the theme styling responsible for the title bar, tabs bar, and context menu, then verify that Light differs from System and restores the earlier white appearance.

Written by the indexing model from the issue text.