[MRG] Default pygment color for doc

[rflamary]

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

• Change the code color scheme back to default (green background is ugly)
• Add the static files back in a better folder for the introduction to OT in the gallery.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.