Show preview of the selected theme

[NEBANU]

Solves issue 10 in the JabRef Themes Repo: JabRef/themes.jabref.org#10
It added a preview of the light and dark theme when the user selects it.
The preview is removed if the user does not change their theme or chooses a custom them.
Dark Theme Preview:

Light Theme Preview:

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

Your code currently does not meet JabRef's code guidelines.
We use Checkstyle to identify issues.
The tool reviewdog already placed comments on GitHub to indicate the places. See the tab "Files" in you PR.
Please carefully follow the setup guide for the codestyle.
Afterwards, please run checkstyle locally and fix the issues.

You can check review dog's comments at the tab "Files changed" of your pull request.

[koppor]

Remove AI generated comments.

Remove image (not linked in the docs)

[koppor]

Wrong format.

[Siedlerchr]

Does not solve the original issue and does not work for custom themes, although I like the idea