For #12877: Make Reader mode default to set theme

[fice-t]

This makes Reader mode check the current dark mode status and default to the dark theme if either the system or app-level dark mode is active.

Sample: using Fenix

Device-2022-09-27-150312-reencoded.webm

Note: Currently Fenix doesn't refresh Reader Mode for the selected tab (the View?) when changing the theme via the Fenix settings menu, meaning the user has to re-select the tab to see the effect. I'll file a bug over there once this is merged.

Though the above Fenix bug could be worked around if ActiveReaderViewContentMessageHandler had more than a weak reference to its config property.

Pull Request checklist

• [x] Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• [x] Tests: This PR includes thorough tests or an explanation of why it does not
• [x] Changelog: This PR includes a changelog entry or does not need one
• [x] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues closed by this pull request are added to the milestone of the version currently in development.
• Breaking Changes: If this is a breaking change, please push a draft PR on Reference Browser to address the breaking issues.

GitHub Automation

Fixes #12877 Fixes #3289

[mergify[bot]]

This pull request has conflicts when rebasing. Could you fix it @fice-t? 🙏