Loic: Update theme and prepare for Dotorg submission

[beafialho]

Changes from WordPress Playground:

• Replaces screenshot with one with the right size
• Adjusts spacing between columns in the theme's templates
• Adjusts pullquote and code block
• Sets different color to headings (and corrects headings color in footer)
• Removes wrong tags as pointed out by Theme Check plugin

Also exported as a zip file.

• Preview loaded from this PR – available before this PR is merged
• Preview loaded from the main branch – available after this PR is merged