User theme (light or dark mode) should be an accessible variable

[AlexandreSajus]

Description

A user on Discord requested this. The user's theme (light or dark mode) should be a callable variable within the code. This way, the dev could have different images displayed according to the current theme (a white background image in light mode and a black background image in dark mode)

Acceptance Criteria

• [ ] Ensure new code is unit tested, and check code coverage is at least 90%.
• [ ] Create related issue in taipy-doc for documentation and Release Notes.
• [ ] Check if a new demo could be provided based on this, or if legacy demos could be benefit from it.
• [ ] Ensure any change is well documented.

Code of Conduct

• [X] I have checked the existing issues.
• [ ] I am willing to work on this issue (optional)