Round the avatars and light them for the theme

Round the avatars and light them for the theme

The tiles are rounded squares now: the element's border-radius does the
clip, so DiceBear's circular mask and the document-wide id it carried are
gone. That also makes a mark about 130 bytes smaller.

The disc in the sky follows the theme. A dark theme gets the moon: pale
cream, near-white and a cool grey. A light theme keeps the sun: cream,
yellow and sunset orange. Switching theme in the account page redraws
the avatars in place, because theme.js writes data-theme on <html> and
the bundle watches that attribute; following the system is the same path
because theme.js resolves it to a concrete variant.

Rebuilt the bundle (10KB, 4KB gzipped) and rendered its real output at
20px and 52px on both themes with the rounded clip applied: the moon
reads against the darker skies and the sun still reads on the light one.