Deskotp: Legacy Markdown Editor: Fix styles in seconary windows and remove red focus-visible border

[personalizedrefrigerator]

Summary

This pull request:

• Fixes an issue where legacy Markdown editor style overrides (e.g. font size, heading styles, etc.) weren't loaded in secondary windows.
• Fixes an issue where the legacy Markdown editor had an orange :focus-visible border when spellcheck was enabled.

See the relevant forum post.

Note

This pull request fixes a regression and thus targets release-3.2.

Testing plan

1. Start Joplin.
2. Enable the legacy Markdown Editor.
3. Enable spellcheck.
4. Open a note in a new window.
5. Focus the editor.
6. Verify that no orange border is shown around the editor.
7. Add a heading.
8. Verify that the heading is in a larger font.