Add per-user page tone modes to reader

public/read.html

@@ -38,6 +38,7 @@
       <button data-mode="page" title="One page at a time">Page</button>
       <button data-mode="book" title="Two pages side by side, like an open book">Book</button>
     </div>
+    <button id="page-tone" class="btn page-tone" type="button" title="Change page colors; controls and theme stay the same" aria-label="Page tone: Original. Change page colors">Tone: Original</button>
     <button id="shift" class="btn" title="Page pairs look wrong? Shift them by one page">⇄<span class="wide-only"> Shift</span></button>
     <div class="seg zoom-controls" role="group" aria-label="Page zoom">
       <button id="zoomout" title="Zoom out 5%" aria-label="Zoom out 5%">−</button>