UI/Qt: Set dark theme as reported by Qt

[shlyakpavel]

Previously, we did a bit of guessing.
That approach was hacky and less reliable.

[yyny]

Nit:

That approach was hacky and less reliable.

[tcl3]

This doesn't compile for me. According to the docs this property was added in 6.5. On Ubuntu, I have 6.4.2 installed.

It looks like we do use #if (QT_VERSION > QT_VERSION_CHECK(major, minor, patch)) elsewhere in the code, so you could use that to ensure you only use the built in way of doing things if the QT_VERSION is new enough.

[shlyakpavel]

@tcl3 elsewhere, we're using QT_VERSION_CHECK(6, 6, 0) and higher. Are you sure it's a good idea to support ancient Qt versions?

[ladybird-bot]

Hello!

One or more of the commit messages in this PR do not match the Ladybird code submission policy, please check the lint_commits CI job for more details on which commits were flagged and why.
Please do not close this PR and open another, instead modify your commit message(s) with git commit --amend and force push those changes to update this PR.

[tcl3]

This works very nicely on MacOS (with Qt 6.7), where the page will now update live in response to changes in the system color theme 🎉