Firefox 152 breakage (active tabs + blank Private Tabs)

### Describe the bug

As of updating to Firefox v152 today, there seems to be much more breakage than usual when using the Photon theme:

- Wrong background colour on private tabs (probably due to upstream breakage, but I find this change quite jarring when opening a new private window/tab).
- Incorrect active tab colour
- Other small bugs that have built up over the ~2 months this project has been unmaintained for (I don't care as much about these)

### Expected behavior

The Photon theme applies correctly.

### Screenshots

LibreWolf v152.0-1, open to a new Private Tab, showing both issues:

LibreWolf v147.0.4-1, open to the same new Private Tab. Both issues aren't present:

(ignore the pixel density differences, the second screenshot was taken on a much older and less-used laptop)

(also, does GitHub round the corners of uploaded images now?!)

### OS

Windows 10

### OS - Others

_No response_

### Firefox Version

LibreWolf v152.0-1

### 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
- [x] Dark
- [ ] Alpenglow (System: Light)
- [ ] Alpenglow (System: Dark)
- [ ] 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

[Dark Magic](https://addons.mozilla.org/en-US/firefox/addon/nicothin-dark-magic/) theme.

### `user.js` setup

user.js setup

```javascript
// NOTE: (most of) these settings aren't included with Lepton directly - they're personal preferences (or reverting LibreWolf changes), but thought I'd include them for completeness
user_pref("apz.overscroll.enabled", false);
user_pref("browser.gesture.swipe.left", "cmd_scrollLeft");
user_pref("browser.gesture.swipe.right", "cmd_scrollRight");
user_pref("browser.tabs.groups.enabled", false);
user_pref("browser.tabs.hoverPreview.enabled", false);
user_pref("browser.urlbar.trimURLs", false);
user_pref("signon.rememberSignons", true);
user_pref("svg.context-properties.content.enabled", true);
user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", true);

// The actual Lepton user.js content
user_pref("browser.compactmode.show", true);
user_pref("browser.newtabpage.activity-stream.improvesearch.handoffToAwesomebar", false);
user_pref("layout.css.has-selector.enabled", true);
user_pref("userChrome.tab.connect_to_window", true);
user_pref("userChrome.tab.color_like_toolbar", true);
user_pref("userChrome.tab.lepton_like_padding", false);
user_pref("userChrome.tab.photon_like_padding", true);
user_pref("userChrome.tab.dynamic_separator", false);
user_pref("userChrome.tab.static_separator", true);
user_pref("userChrome.tab.static_separator.selected_accent", false);
user_pref("userChrome.tab.bar_separator", false);
user_pref("userChrome.tab.newtab_button_like_tab", false);
user_pref("userChrome.tab.newtab_button_smaller", true);
user_pref("userChrome.tab.newtab_button_proton", false);
user_pref("userChrome.icon.panel_full", false);
user_pref("userChrome.icon.panel_photon", true);
user_pref("userChrome.tab.box_shadow", false);
user_pref("userChrome.tab.bottom_rounded_corner", false);
user_pref("userChrome.tab.photon_like_contextline", true);
user_pref("userChrome.rounding.square_tab", true);
user_pref("userChrome.padding.first_tab", true);
user_pref("userChrome.padding.first_tab.always", true);
user_pref("userChrome.padding.drag_space", true);
user_pref("userChrome.padding.drag_space.maximized", true);
user_pref("userChrome.compatibility.theme", true);
user_pref("userChrome.compatibility.os", true);
user_pref("userChrome.theme.built_in_contrast", true);
user_pref("userChrome.theme.system_default", true);
user_pref("userChrome.theme.proton_color", true);
user_pref("userChrome.theme.proton_chrome", true);
user_pref("userChrome.theme.fully_color", true);
user_pref("userChrome.theme.fully_dark", true);
user_pref("userChrome.decoration.cursor", true);
user_pref("userChrome.decoration.field_border", true);
user_pref("userChrome.decoration.download_panel", true);
user_pref("userChrome.decoration.animate", true);
user_pref("userChrome.padding.tabbar_width", true);
user_pref("userChrome.padding.tabbar_height", true);
user_pref("userChrome.padding.toolbar_button", true);
user_pref("userChrome.padding.navbar_width", true);
user_pref("userChrome.padding.urlbar", true);
user_pref("userChrome.padding.bookmarkbar", true);
user_pref("userChrome.padding.infobar", true);
user_pref("userChrome.padding.menu", true);
user_pref("userChrome.padding.bookmark_menu", true);
user_pref("userChrome.padding.global_menubar", true);
user_pref("userChrome.padding.panel", true);
user_pref("userChrome.padding.popup_panel", true);
user_pref("userChrome.tab.multi_selected", true);
user_pref("userChrome.tab.unloaded", true);
user_pref("userChrome.tab.letters_cleary", true);
user_pref("userChrome.tab.close_button_at_hover", true);
user_pref("userChrome.tab.sound_hide_label", true);
user_pref("userChrome.tab.sound_with_favicons", true);
user_pref("userChrome.tab.pip", true);
user_pref("userChrome.tab.container", true);
user_pref("userChrome.tab.crashed", true);
user_pref("userChrome.fullscreen.overlap", true);
user_pref("userChrome.fullscreen.show_bookmarkbar", true);
user_pref("userChrome.icon.library", true);
user_pref("userChrome.icon.panel", true);
user_pref("userChrome.icon.menu", true);
user_pref("userChrome.icon.context_menu", true);
user_pref("userChrome.icon.global_menu", true);
user_pref("userChrome.icon.global_menubar", true);
user_pref("userChrome.icon.1-25px_stroke", true);
user_pref("userContent.player.ui", true);
user_pref("userContent.player.icon", true);
user_pref("userContent.player.noaudio", true);
user_pref("userContent.player.size", true);
user_pref("userContent.player.click_to_play", true);
user_pref("userContent.player.animate", true);
user_pref("userContent.newTab.full_icon", true);
user_pref("userContent.newTab.animate", true);
user_pref("userContent.newTab.pocket_to_last", true);
user_pref("userContent.newTab.searchbar", true);
user_pref("userContent.page.field_border", true);
user_pref("userContent.page.illustration", true);
user_pref("userContent.page.proton_color", true);
user_pref("userContent.page.dark_mode", true);
user_pref("userContent.page.proton", true);
```

### Additional context

Very likely caused by upstream breakage.

Start by reproducing the Photon-style theme on LibreWolf/Firefox 152 with the reported userChrome settings, focusing on a blank Private Tab and the active tab. Compare the result with Firefox 147 and the supplied screenshots, then trace the affected theme styles. Done means private-tab backgrounds and active-tab colours match the expected Photon appearance without regressing the older version.

Written by the indexing model from the issue text.