Update most Reader Mode labels

Still to do (as a follow up): font size labels and omnibox button labels.

Bug: 1041918
Change-Id: I99c1c814accd079e15dd8861246631a570443a57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007797
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#734351}

components/dom_distiller/core/html/dom_distiller_viewer.html

@@ -20,13 +20,13 @@ found in the LICENSE file.
     <div id="main-content">
       <div id="settings-container" class="desktop-only">
         <button id="settings-toggle" type="button"
-            aria-label="Adjust appearance settings" tabindex=0>
+            aria-label="Customize appearance" tabindex=0>
           <i class="material-icons">text_format</i>
         </button>
         <dialog id="settings-dialog">
           <div id="settings-header">
-            <h2>Reader Mode Styling</h2>
-            <button id="close-settings-button" type="button" aria-label="Close settings"
+            <h2>Customize appearance</h2>
+            <button id="close-settings-button" type="button" aria-label="Close"
                 tabindex=0>
               <i class="material-icons">close</i>
             </button>
@@ -34,23 +34,23 @@ found in the LICENSE file.
           <div id="settings-fields">
             <div class="setting">
               <label id="font-family-label" class="screen-reader-only">
-                Font Style
+                Font style
               </label>
               <select id="font-family-selection" aria-labelledby="font-family-label" name="font-family">
-                <option value="sans-serif" selected>Sans-serif font</option>
-                <option value="serif">Serif font</option>
-                <option value="monospace">Monospace font</option>
+                <option value="sans-serif" selected>sans-serif font</option>
+                <option value="serif">serif font</option>
+                <option value="monospace">monospace font</option>
               </select>
             </div>
             <div class="setting">
               <fieldset id="theme-selection">
                 <legend class="screen-reader-only">
-                  Page Theme
+                  Page Color
                 </legend>
                 <ul>
                   <li class="theme-option">
                     <label id="theme-option-light-label" class="screen-reader-only">
-                      Light
+                      light
                     </label>
                     <input type="radio" name="theme" value="light"
                        aria-labelledby="theme-option-light-label"
@@ -59,7 +59,7 @@ found in the LICENSE file.
                   </li>
                   <li class="theme-option">
                     <label id="theme-option-sepia-label" class="screen-reader-only">
-                      Sepia
+                      sepia
                     </label>
                     <input type="radio" name="theme" value="sepia"
                        aria-labelledby="theme-option-sepia-label"
@@ -68,7 +68,7 @@ found in the LICENSE file.
                   </li>
                   <li class="theme-option">
                     <label id="theme-option-dark-label" class="screen-reader-only">
-                      Dark
+                      dark
                     </label>
                     <input type="radio" name="theme" value="dark"
                        aria-labelledby="theme-option-dark-label"