Support embeddings on dark background

[farisd16]

This PR removes the white background from embeddings in case the editor was in dark mode at the time of sharing the diagram.

[FelixTJDietrich]

Test:

[FelixTJDietrich]

This is how it looks when I embed it in dark mode but view it in light mode on GitHub:

See for yourself in my comment.
You would need to have a black line with white outline or the other way around. Or like a black and white line next to each other so you can see it regardless of the appearance selected on GitHub.

Please also add testing instructions and test if for yourself with the test server. You can deploy your branch here https://github.com/ls1intum/Apollon_standalone/actions via the Deploy to Dev workflow.

[farisd16]

The intention was to export the diagram for dark/light mode depending on the mode currently used in the standalone Apollon editor. I agree that is not an optimal solution and can lead to confusion. I will close the PR therefore.