Multiple Themes: Clean up top margin when header & homepage title are hidden

[kjellr]

Followup from #2961.

When both the "Hide Header" and "Hide Homepage Title" settings are activated, many of Varia's child themes needed some slight adjustments to ensure:

• That the first child block of entry-content had top margin, and didn't bump up against the top of the screen.
• That certain full-width blocks do properly bump up against the top of the screen.

The fix was generally just to:

• Add a rule to Varia to ensure that the top margin is present by default.
• Recompile CSS for all of the child themes.
• Add special case margins for themes that use non-standard margins (Shawburn, Rockfield, Morden, Redhill).

Screenshots

Generally, this is what things looked like before, using Barnsbury (on the left) and Stow (on the right) as examples:

Before

Full-width Media/Text Block	Paragraph

After

Full-width Media/Text Block	Paragraph

[scruffian]

LGTM