[ui][plugin manager] Fix broken color palette when first opening the plugin manager on non-default themes

[nirvn]

Description

Fixes this horror when first opening the plugin manager:

The core of the issue is that the plugin manager is initiated before QGIS applies the custom theme, resulting in using the wrong color palette.

While at it, I've upgraded the stylesheet to rely on palette() as it's just a cleaner way to go at it.

AI tool usage

• AI tool(s) (Copilot, Claude, or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated. Failure to be honest might result in banning.

[github-actions]

🪟 x64-windows — ❌ 1 port(s) failed

Failed: py-pandas

📦 py-pandas

_{from vcpkg-manifest-install.log (portfile-execute abort)}

  scripts/cmake/vcpkg_from_github.cmake:120 (z_vcpkg_download_distfile)
  C:/Users/runneradmin/AppData/Local/vcpkg/registries/git-trees/910f6fd1f988c66cd5c4ee3290ff174cdbd81cf7/portfile.cmake:19 (vcpkg_from_github)
  scripts/ports.cmake:209 (include)


error: building py-pandas:x64-windows-release failed with: BUILD_FAILED
See https://learn.microsoft.com/vcpkg/troubleshoot/build-failures?WT.mc_id=vcpkg_inproduct_cli for more information.
Elapsed time to handle py-pandas:x64-windows-release: 5.2 s
Please ensure you're using the latest port files with `git pull` and `vcpkg update`.
Then check for known issues at:
  https://github.com/microsoft/vcpkg/issues?q=is%3Aissue+is%3Aopen+in%3Atitle+py-pandas
You can submit a new issue at:
  https://github.com/microsoft/vcpkg/issues/new?title=%5Bpy-pandas%5D%20build%20error%20on%20x64-windows-release&body=Copy%20issue%20body%20from%20collapsed%20section%20%22issue_body.md%22%20in%20the%20ci%20log%20output
::group::issue_body.md
Package: py-pandas:x64-windows-release@3.0.5

**Host Environment**

- Host: x64-windows
- Compiler: MSVC 19.44.35228.0
- CMake Version: 4.4.0
-    vcpkg-tool version: 2026-07-27-98d7cb0cf1f4686a3e43aa5672b6230c1d56bce8
    vcpkg-scripts version: unknown

**To Reproduce**

`vcpkg install --x-feature=3d --x-feature=auth --x-feature=bindings --x-feature=exiv2 --x-feature=gui --x-feature=pdal --x-feature=qtpositioning --x-feature=sfcgal`

**Failure logs**

Trying to download meson-1.9.0.tar.gz using asset cache https://assetcache.open-vcpkg.org/assetcache/ecd69b6734be14c467f7db67dd88c0e57ebfad83ce3ddada131ff3e43ac964523e1083d7c7f3803033a9a76adbc32ad26dd2e3aca69884269000ca64130bde07
Download successful! Asset cache hit, did not try authoritative source https://github.com/mesonbuild/meson/archive/1.9.0.tar.gz
-- Applying patch D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/share/vcpkg-tool-meson/meson-intl.patch
-- Applying patch D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/share/vcpkg-tool-meson/adjust-python-dep.patch
-- Applying patch D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/share/vcpkg-tool-meson/adjust-args.patch
-- Applying patch D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/share/vcpkg-tool-meson/remove-pkgconfig-specialization.patch
-- Applying patch D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/share/vcpkg-tool-meson/meson-56879d5.diff
Added vcpkg bin path to search for dlls. Path added: D:\a\QGIS\QGIS\build\vcpkg_installed\x64-windows-release\bin
-- Found Python version '3.12.11 at D:/a/QGIS/QGIS/build/vcpkg_installed/x64-windows-release/tools/python3/python.exe'

</details>

📦 [Download `failing-port-logs-x64-windows` artifact](https://github.com/qgis/QGIS/actions/runs/35305218734/artifacts/10530618559) &middot; 📑 [Download full `build-logs-x64-windows` artifact](https://github.com/qgis/QGIS/actions/runs/35305218734/artifacts/10530653647) &middot; 🔧 [Workflow run](https://github.com/qgis/QGIS/actions/runs/35305218734) &middot; 📌 [Commit `1c01493`](https://github.com/qgis/QGIS/commit/1c0149304187ab6a59b6a10748b3769f250a1a4f)
<!-- ====endsection vcpkg-failures-x64-windows-->

<!-- ====startsection macos-qt6-->
### 🍎 MacOS Qt6 builds
Download [MacOS Qt6 builds of this PR for testing](https://github.com/qgis/QGIS/actions/runs/35305218738/artifacts/10538867458).
_This app is not notarized, run `sudo xattr -d com.apple.quarantine /Applications/QGIS*.app` to avoid the warning_
*(Built from commit 1c0149304187ab6a59b6a10748b3769f250a1a4f)*

<!-- ====endsection macos-qt6-->