[themes] convert to grid layout for mobile

[discomrade]

This commit changes the themes to use a grid layout, inspired by Danbooru. This keeps the regular view the same, but changes to a narrow single-column view for mobile screens, showing more of the main content.

The index page changes from this into this [ . . . ]

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The view page changes from this into this

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The next steps after this are to add an anchor link to scroll down to the search field, and to retract the nav menu into a toggle menu, for themes where that is needed.