Instant Search: Fix up theme issues

[gibrown]

• storefront theme (the default for all WooCommerce), has a number of broken styling details
• storefront: the text color in the close button is black (maybe this won't matter when we change to an X) http://gibrown.wpsandbox.me/demo/?blog_id=20115252&theme=storefront&s=related%20psts
• 2014 no load more button at bottom - http://gibrown.wpsandbox.me/demo/?s=related%20psts&blog_id=20115252&theme=twentyfourteen
• 2015 weird break on filters http://gibrown.wpsandbox.me/demo?blog_id=20115252&theme=twentyfifteen&s=related%20psts https://cloudup.com/cy-YEMc6bwz

[keoshi]

• storefront theme (the default for all WooCommerce), has a number of broken styling details

Do you remember what these were, @gibrown ? I can only see the poor legibility on the widget titles here:

If that's it, I think that a good case to extend a similar solution to what I proposed on #14666 to a few elements, which also applies to the second bullet point on this issue.

[keoshi]

• 2014 no load more button at bottom - http://gibrown.wpsandbox.me/demo/?s=related%20psts&blog_id=20115252&theme=twentyfourteen

Addressed in #14669

[keoshi]

• 2015 weird break on filters http://gibrown.wpsandbox.me/demo?blog_id=20115252&theme=twentyfifteen&s=related%20psts https://cloudup.com/cy-YEMc6bwz

Addressed in #14670

[gibrown]

I think all of the storefront stuff has now been fixed by the styling changes for the search box, close button, etc.