First prototype of an improved theme/style

[gmantele]

This is kind of a draft now. There is clearly matter of improvement (especially to make it render better on small screens like smartphone). Few things can still be fixed before merging.

Fixes #6

[molinaro-m]

Style pointers now look OK.
I think there's still place for a better responsiveness, but that will come later on.

[JeremyMcCormick]

This is a huge improvement - thanks for this update!

My requested changes would be:

• Improve the text in the carousel. The title is too big and the author too small.
• Make a row including the carousel and newsletter together (implicit in this suggestion is decreasing the size of the carousel which seems too big now)
• Have the prose content on the landing page be on its own row
• Change footer labels like "About IVOA" to grey text
• Change the now light blue footer links to white

Following discussion, these changes could be incorporated on separate PRs if you wish.

(These are mostly just my preferences.)

[JeremyMcCormick]

I am going to leave it up to you what changes to include in this PR based on my feedback (see checklist).

Additional improvements we decide to make in this area can be implemented on separate issues.