{#- The theme control, the landing page's (`docs/landing/src/shell.html`): one round button
    with two states and no third (WP-2021).  The page follows the system until a click pins
    the other theme.  pydata's own switcher is a three-item menu, so this replaces it.

    The glyph is CSS: the sun and the moon are `only-light` and `only-dark`, which pydata
    hides by `html[data-theme]`.  The click is `_static/rietx-theme.js`, which writes the
    keys pydata reads, so the theme engine underneath is still pydata's.  Hidden without
    script, as pydata's is, by `pst-js-only`. -#}
<button class="rietx-theme pst-js-only" type="button" aria-label="Switch to the dark theme" title="Switch to the dark theme">
  <svg class="only-light" viewBox="0 0 24 24" aria-hidden="true"><circle cx="12" cy="12" r="4"/><path d="M12 2v2M12 20v2M4.9 4.9l1.4 1.4M17.7 17.7l1.4 1.4M2 12h2M20 12h2M4.9 19.1l1.4-1.4M17.7 6.3l1.4-1.4"/></svg>
  <svg class="only-dark" viewBox="0 0 24 24" aria-hidden="true"><path d="M20.5 14.6A8.6 8.6 0 0 1 9.4 3.5 8.6 8.6 0 1 0 20.5 14.6z"/></svg>
</button>