Adds CSS variables for the highlight and shadow so they don't need
to be custom-defined for each color of button.
Elements should use more generic classes like green_button instead of
add_tag_button unless there's a good reason.
Appearing on search and album pages, the tray is where you can
remove items from your clipboard without having to click on its
checkbox -- that photo card may not even be on the current page.
voussoir