Tweak theme preview

This update will generate in the server's startup routine unique screenshots of the landing page for each theme and each language.

If updates to the landing page occur or additional languages will be added there is no need to regenerate the screenshots manually.

When a user visits the theme switcher page the screenshots of the landing page he will see will depend on his selected language.

Additional the bad code style where the keyframe animation of the landing page was generated via javascript is fixed.