Add the # on `header_color`.

This is stored without a #, which breaks the CSS and ignores the set header_color.
Remove searchform.php to use default form
Remove duplicate title attributes
Pass title HTML to `the_title`, so it only displays when there is a post title.
Move screen reader text to the very-top of the page
Use a `button` for the menu toggle, so it can be accessed by keyboard nav
Make headings more semantic

Site description should not be a heading, and post
titles should be h2s, when in a list-view. See #28
Change colors for WCAG contrast rules, see #28
Add focus styling for all links
Menu toggle needs special focus state.