System, Dark and Light theme problems

Thank you for creating a fast native note-taking app with sync that just works! Here is the bug report:

### Expected
Properly themed elements based on selected theme setting and system theme combination.

### Observed
Mixture of dark and light themed elements in some settings:
- System Appearance + Dark theme on system side: Works as expected
- System Appearance + Light theme on system side: Sidebar and markdown preview is improperly styled as Dark.
- Dark in SimpleNote + Light theme on system side: Popup notes don't respect app setting (background uses system setting) except markdown preview text and table color.
- Light in SimpleNote + Light theme on system side: Works as expected
- Dark in SimpleNote + Dark theme on system side: Works as expected
- Light in SimpleNote + Dark theme on system side: Popup notes don't respect app setting (background uses system setting) except markdown preview text and table color.

See screenshots at the bottom.

### Reproduced
Consistent between app launches; behaves the same way regardless of prior system setting.
- Set the app's theme in its Settings and your system theme based on problematic combinations mentioned above.
- I used AppStore version of SimpleNote (2.21, latest) on macOS Ventura 13.6.4.


Reproduce the reported theme combinations using the app's Settings on macOS, focusing on the sidebar, markdown preview, and popup notes. Trace the theme handling for those elements and verify that each consistently follows the selected app theme rather than the system theme.

Written by the indexing model from the issue text.