PDF: Tweak font-size and colors to more closely match other MD WebUI

chrome/browser/resources/pdf/elements/viewer-zoom-toolbar/viewer-zoom-button.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 #wrapper {
   transition: transform 250ms;
   transition-timing-function: cubic-bezier(0, 0, 0.2, 1);
 }
 
 :host([closed]) #wrapper {
   /* 132px roughly flips the location of the button across the right edge of the
    * page. */
   transform: translateX(132px);
   transition-timing-function: cubic-bezier(0.4, 0, 1, 1);
 }
 
 :host-context([dir=rtl]):host([closed]) #wrapper {
   transform: translateX(-132px);
 }
 
 paper-fab {
   --paper-fab-keyboard-focus-background: var(--viewer-icon-ink-color);
   --paper-fab-mini: {
     height: 36px;
     padding: 8px;
     width: 36px;
   };
   @apply(--shadow-elevation-4dp);
   background-color: rgb(242, 242, 242);
-  color: rgb(96, 96, 96);
+  color: var(--paper-grey-700);
   overflow: visible;
 }